diff --git a/web/pnpm-lock.yaml b/web/pnpm-lock.yaml
index 1f1d842..e52ac0d 100644
--- a/web/pnpm-lock.yaml
+++ b/web/pnpm-lock.yaml
@@ -119,6 +119,9 @@ importers:
       emoji-mart:
         specifier: ^5.6.0
         version: 5.6.0
+      lib0:
+        specifier: ^0.2.99
+        version: 0.2.117
       lucide-react:
         specifier: ^0.469.0
         version: 0.469.0(react@18.3.1)
diff --git a/web/standalone/package.json b/web/standalone/package.json
index 813a762..aa8720d 100644
--- a/web/standalone/package.json
+++ b/web/standalone/package.json
@@ -29,6 +29,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "emoji-mart": "^5.6.0",
+    "lib0": "^0.2.99",
     "lucide-react": "^0.469.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
diff --git a/web/standalone/src/wasm/collab/gateway.test.ts b/web/standalone/src/wasm/collab/gateway.test.ts
new file mode 100644
index 0000000..60e50e5
--- /dev/null
+++ b/web/standalone/src/wasm/collab/gateway.test.ts
@@ -0,0 +1,337 @@
+import * as decoding from "lib0/decoding";
+import * as encoding from "lib0/encoding";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import * as syncProtocol from "y-protocols/sync";
+import * as Y from "yjs";
+import {
+  parseGatewayClientMsg,
+  tagGatewayFrame,
+  untagGatewayFrame,
+  type GatewayClientMsg,
+} from "@pcbjam/shared";
+import { CollabSubRejectedError, GatewayDocFacade } from "./gateway";
+
+/**
+ * The gateway facade is a mini y-websocket client over one shared multiplexed
+ * socket (load-path-rework 0003 §6). These tests script the socket directly:
+ * what a passive vs active subscription puts on the wire is the laziness
+ * contract — a passive warm-pool sheet must never emit a doc frame.
+ */
+
+// --- scripted WebSocket ------------------------------------------------------
+
+class FakeWebSocket {
+  static instances: FakeWebSocket[] = [];
+  static OPEN = 1;
+  static CLOSED = 3;
+  readyState = 0;
+  binaryType = "blob";
+  sent: Array<string | ArrayBuffer | Uint8Array> = [];
+  onopen: (() => void) | null = null;
+  onmessage: ((e: { data: unknown }) => void) | null = null;
+  onclose: (() => void) | null = null;
+  onerror: (() => void) | null = null;
+  constructor(public url: string) {
+    FakeWebSocket.instances.push(this);
+  }
+  send(data: string | ArrayBuffer | Uint8Array): void {
+    this.sent.push(data);
+  }
+  close(): void {
+    this.readyState = FakeWebSocket.CLOSED;
+    this.onclose?.();
+  }
+  // test drivers
+  open(): void {
+    this.readyState = FakeWebSocket.OPEN;
+    this.onopen?.();
+  }
+  receiveText(text: string): void {
+    this.onmessage?.({ data: text });
+  }
+  receiveFrame(ch: number, frame: Uint8Array): void {
+    const tagged = tagGatewayFrame(ch, frame);
+    const buf = new ArrayBuffer(tagged.length);
+    new Uint8Array(buf).set(tagged);
+    this.onmessage?.({ data: buf });
+  }
+  drop(): void {
+    this.readyState = FakeWebSocket.CLOSED;
+    this.onclose?.();
+  }
+  controls(): GatewayClientMsg[] {
+    return this.sent
+      .filter((d): d is string => typeof d === "string")
+      .map((d) => parseGatewayClientMsg(d)!)
+      .filter(Boolean);
+  }
+  frames(): Array<{ ch: number; type: number; frame: Uint8Array }> {
+    return this.sent
+      .filter((d): d is ArrayBuffer | Uint8Array => typeof d !== "string")
+      .map((d) => {
+        const bytes = d instanceof ArrayBuffer ? new Uint8Array(d) : d;
+        const tagged = untagGatewayFrame(bytes)!;
+        const dec = decoding.createDecoder(tagged.frame.slice());
+        return {
+          ch: tagged.ch,
+          type: decoding.readVarUint(dec),
+          frame: tagged.frame.slice(),
+        };
+      });
+  }
+}
+
+vi.stubGlobal("WebSocket", FakeWebSocket);
+
+let projSeq = 0;
+function facadeOpts(docPath: string, passive = false) {
+  return {
+    endpoint: "http://localhost:3055",
+    scopeId: "scope-x",
+    // Fresh project per call site keeps the module-global connection registry
+    // from leaking one test's socket into the next.
+    projectId: `proj-${projSeq}`,
+    docPath,
+    passive,
+  };
+}
+
+function newProject(): void {
+  projSeq++;
+}
+
+const step2From = (serverDoc: Y.Doc, clientStep1?: Uint8Array): Uint8Array => {
+  // Answer a client Step1 the way a BoardRoom would.
+  let sv: Uint8Array | undefined;
+  if (clientStep1) {
+    const dec = decoding.createDecoder(clientStep1.slice());
+    decoding.readVarUint(dec); // MESSAGE_SYNC
+    decoding.readVarUint(dec); // messageYjsSyncStep1
+    sv = decoding.readVarUint8Array(dec);
+  }
+  const enc = encoding.createEncoder();
+  encoding.writeVarUint(enc, 0);
+  syncProtocol.writeSyncStep2(enc, serverDoc, sv);
+  return encoding.toUint8Array(enc);
+};
+
+const cleanups: Array<() => void> = [];
+afterEach(() => {
+  for (const c of cleanups.splice(0)) c();
+  FakeWebSocket.instances = [];
+});
+
+function track(facade: GatewayDocFacade, ...docs: Y.Doc[]): void {
+  cleanups.push(() => {
+    try {
+      facade.destroy();
+    } catch {
+      /* already destroyed */
+    }
+    for (const d of docs) d.destroy();
+  });
+}
+
+describe("gateway facade — active documents", () => {
+  it("subscribes, syncs via Step1/Step2, and pushes local updates", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("a.kicad_sch"));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    expect(ws.url).toContain("/parties/project-room/project%3Ascope-x%3A");
+
+    ws.open();
+    const [sub] = ws.controls();
+    expect(sub).toEqual({
+      t: "sub",
+      ch: expect.any(Number) as number,
+      doc: "a.kicad_sch",
+      mode: "active",
+    });
+    const step1 = ws.frames().find((f) => f.type === 0);
+    expect(step1).toBeTruthy();
+
+    // Server state lands via Step2 → whenSynced resolves, doc holds it.
+    const serverDoc = new Y.Doc();
+    serverDoc.getMap("m").set("k", "v");
+    cleanups.push(() => serverDoc.destroy());
+    ws.receiveFrame(sub!.ch, step2From(serverDoc, step1!.frame));
+    await facade.whenSynced();
+    expect(doc.getMap("m").get("k")).toBe("v");
+
+    // A local edit goes out as a sync update frame.
+    ws.sent = [];
+    doc.getMap("m").set("mine", 1);
+    expect(ws.frames().some((f) => f.type === 0)).toBe(true);
+  });
+
+  it("re-sends Step1 on a resync prompt", () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("a.kicad_sch"));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    const ch = ws.controls()[0]!.ch;
+    ws.sent = [];
+    ws.receiveText(JSON.stringify({ t: "resync", ch }));
+    expect(ws.frames().filter((f) => f.type === 0).length).toBe(1);
+  });
+});
+
+describe("gateway facade — passive warm pool (the laziness contract)", () => {
+  it("puts NO doc frame on the wire, and whenSynced resolves on subscribe", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("b.kicad_sch", true));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    expect(ws.controls()[0]).toMatchObject({ t: "sub", mode: "passive" });
+    // Awareness/query frames are allowed (they're not demand); sync is not.
+    expect(ws.frames().filter((f) => f.type === 0)).toEqual([]);
+    await facade.whenSynced(); // resolves without any doc state
+    expect(doc.share.size).toBe(0);
+  });
+
+  it("touched marks dirty via callback; activate() upgrades and truly syncs", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("b.kicad_sch", true));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    const ch = ws.controls()[0]!.ch;
+
+    let touched = 0;
+    facade.onTouched(() => touched++);
+    ws.receiveText(JSON.stringify({ t: "touched", ch }));
+    expect(touched).toBe(1);
+
+    ws.sent = [];
+    const syncing = facade.activate();
+    expect(ws.controls()).toEqual([{ t: "act", ch }]);
+    const step1 = ws.frames().find((f) => f.type === 0);
+    expect(step1).toBeTruthy();
+    const serverDoc = new Y.Doc();
+    serverDoc.getMap("m").set("from", "server");
+    cleanups.push(() => serverDoc.destroy());
+    ws.receiveFrame(ch, step2From(serverDoc, step1!.frame));
+    await syncing;
+    expect(doc.getMap("m").get("from")).toBe("server");
+  });
+
+  it("a local write into a passive doc auto-activates instead of silently desyncing", () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("b.kicad_sch", true));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    ws.sent = [];
+    doc.getMap("m").set("stray", true);
+    expect(ws.controls().some((m) => m.t === "act")).toBe(true);
+    expect(ws.frames().some((f) => f.type === 0)).toBe(true);
+  });
+});
+
+describe("gateway facade — suberr is terminal", () => {
+  it("rejects pending and future syncs with CollabSubRejectedError", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("bad.kicad_sch"));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    const ch = ws.controls()[0]!.ch;
+    const pending = facade.whenSynced();
+    ws.receiveText(
+      JSON.stringify({ t: "suberr", ch, status: 409, message: "flagged" }),
+    );
+    await expect(pending).rejects.toBeInstanceOf(CollabSubRejectedError);
+    await expect(facade.activate()).rejects.toMatchObject({ status: 409 });
+  });
+});
+
+describe("gateway facade — presence channel", () => {
+  it("never speaks sync; publishes and receives awareness", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("~presence"));
+    track(facade, doc);
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    await facade.whenSynced();
+    expect(ws.frames().filter((f) => f.type === 0)).toEqual([]);
+    // Query for peers went out on open.
+    expect(ws.frames().some((f) => f.type === 3)).toBe(true);
+
+    ws.sent = [];
+    facade.awareness.setLocalState({ user: { id: "me" } });
+    expect(ws.frames().some((f) => f.type === 1)).toBe(true);
+
+    // An inbound query is answered with our own state.
+    const ch = 0;
+    ws.sent = [];
+    ws.receiveFrame(ch, new Uint8Array([3]));
+    expect(ws.frames().some((f) => f.type === 1)).toBe(true);
+  });
+});
+
+describe("gateway connection — mux + reconnect", () => {
+  it("facades of one project share one socket with distinct channels", () => {
+    newProject();
+    const docA = new Y.Doc();
+    const docB = new Y.Doc();
+    const a = new GatewayDocFacade(docA, facadeOpts("a.kicad_sch"));
+    const b = new GatewayDocFacade(docB, facadeOpts("b.kicad_sch", true));
+    track(a, docA);
+    track(b, docB);
+    expect(FakeWebSocket.instances.length).toBe(1);
+    const ws = FakeWebSocket.instances[0]!;
+    ws.open();
+    const subs = ws.controls().filter((m) => m.t === "sub");
+    expect(subs.length).toBe(2);
+    expect(new Set(subs.map((m) => m.ch)).size).toBe(2);
+  });
+
+  it("reconnect re-subscribes with the CURRENT mode and re-syncs actives", async () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("a.kicad_sch", true));
+    track(facade, doc);
+    const ws1 = FakeWebSocket.instances.at(-1)!;
+    ws1.open();
+    const ch = ws1.controls()[0]!.ch;
+    // Activate (and settle the sync) on socket 1.
+    const syncing = facade.activate();
+    const step1 = ws1.frames().find((f) => f.type === 0)!;
+    const serverDoc = new Y.Doc();
+    cleanups.push(() => serverDoc.destroy());
+    ws1.receiveFrame(ch, step2From(serverDoc, step1.frame));
+    await syncing;
+
+    vi.useFakeTimers();
+    cleanups.push(() => vi.useRealTimers());
+    ws1.drop();
+    vi.advanceTimersByTime(1_000); // past the reconnect backoff
+    const ws2 = FakeWebSocket.instances.at(-1)!;
+    expect(ws2).not.toBe(ws1);
+    ws2.open();
+    // The once-passive sub comes back as ACTIVE, followed by a fresh Step1.
+    expect(ws2.controls()[0]).toMatchObject({ t: "sub", mode: "active" });
+    expect(ws2.frames().some((f) => f.type === 0)).toBe(true);
+  });
+
+  it("destroying the last facade closes the shared socket", () => {
+    newProject();
+    const doc = new Y.Doc();
+    const facade = new GatewayDocFacade(doc, facadeOpts("a.kicad_sch"));
+    const ws = FakeWebSocket.instances.at(-1)!;
+    ws.open();
+    facade.destroy();
+    doc.destroy();
+    expect(ws.readyState).toBe(FakeWebSocket.CLOSED);
+  });
+});
diff --git a/web/standalone/src/wasm/collab/gateway.ts b/web/standalone/src/wasm/collab/gateway.ts
new file mode 100644
index 0000000..2278ccf
--- /dev/null
+++ b/web/standalone/src/wasm/collab/gateway.ts
@@ -0,0 +1,543 @@
+/**
+ * Client side of the ProjectRoom gateway (load-path-rework 0003): ONE
+ * websocket per (endpoint, project) carrying every per-doc collab room plus
+ * the `~presence` channel, replacing one socket per room. Facades implement
+ * the existing {@link YjsProvider} seam, so sheet-manager / sibling-restage /
+ * cross-app keep working unchanged; the sheet manager's warm pool additionally
+ * uses `passive` subscriptions (register interest, no doc sync) + `activate()`
+ * so parked sheets never wake their BoardRoom (the 0001 §5 lazy amendment).
+ *
+ * The per-facade y-protocol client is the ~150 lines y-partyserver's provider
+ * used to do for us (Step1/Step2/Update via y-protocols/sync, awareness via
+ * y-protocols/awareness), reading/writing frames tagged with the facade's
+ * channel id (@pcbjam/shared gateway-wire).
+ */
+
+import * as decoding from "lib0/decoding";
+import * as encoding from "lib0/encoding";
+import {
+  applyAwarenessUpdate,
+  Awareness,
+  encodeAwarenessUpdate,
+  removeAwarenessStates,
+} from "y-protocols/awareness";
+import * as syncProtocol from "y-protocols/sync";
+import type * as Y from "yjs";
+import {
+  type GatewayClientMsg,
+  type GatewayServerMsg,
+  type GatewaySubMode,
+  parseGatewayServerMsg,
+  PRESENCE_DOC_PATH,
+  projectRoomName,
+  tagGatewayFrame,
+  untagGatewayFrame,
+} from "@pcbjam/shared";
+import { cwarn } from "./debug";
+import type { YjsProvider } from "./provider";
+
+const MESSAGE_SYNC = 0;
+const MESSAGE_AWARENESS = 1;
+const MESSAGE_QUERY_AWARENESS = 3;
+
+const MAX_BACKOFF_MS = 30_000;
+const ACTIVATE_TIMEOUT_MS = 30_000;
+
+/**
+ * The gateway refused a subscription (`suberr`): 409 invalid-file, 403
+ * presence-as-readonly… Terminal for the channel — retrying cannot help until
+ * the underlying condition changes, so callers must NOT enter a retry ladder
+ * (sheet-manager treats it like SexprVersionError).
+ */
+export class CollabSubRejectedError extends Error {
+  constructor(
+    public readonly docPath: string,
+    public readonly status: number,
+    message: string,
+  ) {
+    super(`collab subscription to ${docPath} rejected (${status}): ${message}`);
+    this.name = "CollabSubRejectedError";
+  }
+}
+
+function gatewayWsUrl(endpoint: string, room: string, token?: string): string {
+  let base = endpoint.replace(/\/$/, "");
+  if (!/^[a-z]+:\/\//i.test(base)) {
+    const local = /^(localhost|127\.0\.0\.1|\[::1\])(:|$)/.test(base);
+    base = `${local ? "ws" : "wss"}://${base}`;
+  }
+  base = base.replace(/^http/, "ws");
+  const url = new URL(
+    `${base}/parties/project-room/${encodeURIComponent(room)}`,
+  );
+  url.searchParams.set("_pk", Math.random().toString(36).slice(2, 12));
+  if (token) url.searchParams.set("token", token);
+  return url.toString();
+}
+
+// --- the shared per-project socket ------------------------------------------
+
+class GatewayConnection {
+  private ws: WebSocket | null = null;
+  private open = false;
+  private closed = false;
+  private attempts = 0;
+  private reconnectTimer: ReturnType<typeof setTimeout> | undefined;
+  private chSeq = 0;
+  private readonly facades = new Map<number, GatewayDocFacade>();
+  refs = 0;
+
+  constructor(
+    private readonly endpoint: string,
+    private readonly room: string,
+    private readonly token: string | undefined,
+    private readonly onGone: () => void,
+  ) {
+    this.dial();
+  }
+
+  private dial(): void {
+    if (this.closed) return;
+    let ws: WebSocket;
+    try {
+      ws = new WebSocket(gatewayWsUrl(this.endpoint, this.room, this.token));
+    } catch (err) {
+      cwarn(`[gateway] dial failed for ${this.room}`, err);
+      this.scheduleReconnect();
+      return;
+    }
+    ws.binaryType = "arraybuffer";
+    this.ws = ws;
+    ws.onopen = () => {
+      if (this.closed || this.ws !== ws) return;
+      this.open = true;
+      this.attempts = 0;
+      // Re-establish every live subscription, then let each facade run its
+      // (re)open protocol — sub before Step1, ordered on one socket.
+      for (const facade of this.facades.values()) {
+        this.sendControl(facade.subMsg());
+        facade.handleSocketOpen();
+      }
+    };
+    ws.onmessage = (e) => {
+      if (this.ws !== ws) return;
+      const data: unknown = e.data;
+      if (typeof data === "string") {
+        const msg = parseGatewayServerMsg(data);
+        if (msg) this.facades.get(msg.ch)?.handleControl(msg);
+        return;
+      }
+      if (data instanceof ArrayBuffer) {
+        const tagged = untagGatewayFrame(new Uint8Array(data));
+        if (tagged) this.facades.get(tagged.ch)?.handleFrame(tagged.frame);
+      }
+    };
+    const down = () => {
+      if (this.ws !== ws) return;
+      const wasOpen = this.open;
+      this.open = false;
+      this.ws = null;
+      if (wasOpen) {
+        for (const facade of this.facades.values()) facade.handleSocketDown();
+      }
+      this.scheduleReconnect();
+    };
+    ws.onclose = down;
+    ws.onerror = down;
+  }
+
+  private scheduleReconnect(): void {
+    if (this.closed || this.reconnectTimer !== undefined) return;
+    // Same ladder the per-room provider used (100ms · 2^n, capped) — but ONE
+    // ladder for the whole project instead of one per room.
+    const delay = Math.min(100 * 2 ** this.attempts, MAX_BACKOFF_MS);
+    this.attempts++;
+    this.reconnectTimer = setTimeout(() => {
+      this.reconnectTimer = undefined;
+      this.dial();
+    }, delay);
+  }
+
+  isOpen(): boolean {
+    return this.open;
+  }
+
+  register(facade: GatewayDocFacade): number {
+    const ch = this.chSeq++;
+    this.facades.set(ch, facade);
+    return ch;
+  }
+
+  /** Called by a registered facade once it knows its ch (post-construction). */
+  announce(facade: GatewayDocFacade): void {
+    if (this.open) {
+      this.sendControl(facade.subMsg());
+      facade.handleSocketOpen();
+    }
+  }
+
+  unregister(ch: number): void {
+    if (!this.facades.delete(ch)) return;
+    this.sendControl({ t: "unsub", ch });
+    this.refs--;
+    if (this.refs <= 0) this.shutdown();
+  }
+
+  sendControl(msg: GatewayClientMsg): void {
+    if (this.open && this.ws?.readyState === WebSocket.OPEN) {
+      this.ws.send(JSON.stringify(msg));
+    }
+  }
+
+  sendFrame(ch: number, frame: Uint8Array): void {
+    if (this.open && this.ws?.readyState === WebSocket.OPEN) {
+      this.ws.send(tagGatewayFrame(ch, frame));
+    }
+  }
+
+  private shutdown(): void {
+    this.closed = true;
+    if (this.reconnectTimer !== undefined) {
+      clearTimeout(this.reconnectTimer);
+      this.reconnectTimer = undefined;
+    }
+    try {
+      this.ws?.close();
+    } catch {
+      /* already dead */
+    }
+    this.ws = null;
+    this.open = false;
+    this.onGone();
+  }
+}
+
+const connections = new Map<string, GatewayConnection>();
+
+function acquireConnection(
+  endpoint: string,
+  scopeId: string,
+  projectId: string,
+  token?: string,
+): GatewayConnection {
+  const room = projectRoomName(scopeId, projectId);
+  const key = `${endpoint}|${token ?? ""}|${room}`;
+  let conn = connections.get(key);
+  if (!conn) {
+    const created: GatewayConnection = new GatewayConnection(
+      endpoint,
+      room,
+      token,
+      () => {
+        if (connections.get(key) === created) connections.delete(key);
+      },
+    );
+    conn = created;
+    connections.set(key, conn);
+  }
+  conn.refs++;
+  return conn;
+}
+
+// --- per-doc facade ----------------------------------------------------------
+
+export interface GatewayFacadeOpts {
+  endpoint: string;
+  scopeId: string;
+  projectId: string;
+  docPath: string;
+  token?: string;
+  /** Passive = register interest only (parked warm-pool sheet): no SyncStep1,
+   *  no BoardRoom wake; `touched` hints + awareness still flow. */
+  passive?: boolean;
+}
+
+/**
+ * A YjsProvider backed by one gateway sub-channel. Owns its own Awareness
+ * (skeleton presence publishes a different state per parked room — 0003 §6).
+ */
+export class GatewayDocFacade implements YjsProvider {
+  readonly awareness: Awareness;
+  private readonly conn: GatewayConnection;
+  private readonly ch: number;
+  private readonly isPresence: boolean;
+  private mode: GatewaySubMode;
+  private dead: CollabSubRejectedError | null = null;
+  private destroyed = false;
+  private synced = false;
+  private subEverSent = false;
+  private readonly touchedCbs: Array<() => void> = [];
+  private readonly syncWaiters: Array<{
+    resolve: () => void;
+    reject: (e: unknown) => void;
+  }> = [];
+  private readonly subWaiters: Array<{
+    resolve: () => void;
+    reject: (e: unknown) => void;
+  }> = [];
+
+  constructor(
+    private readonly doc: Y.Doc,
+    opts: GatewayFacadeOpts,
+  ) {
+    this.isPresence = opts.docPath === PRESENCE_DOC_PATH;
+    this.docPath = opts.docPath;
+    this.mode = opts.passive && !this.isPresence ? "passive" : "active";
+    this.awareness = new Awareness(doc);
+    this.conn = acquireConnection(
+      opts.endpoint,
+      opts.scopeId,
+      opts.projectId,
+      opts.token,
+    );
+    this.ch = this.conn.register(this);
+
+    this.awareness.on("update", this.onAwarenessUpdate);
+    if (!this.isPresence) this.doc.on("update", this.onDocUpdate);
+
+    this.conn.announce(this);
+  }
+
+  readonly docPath: string;
+
+  // --- YjsProvider ----------------------------------------------------------
+
+  whenSynced(): Promise<void> {
+    if (this.dead) return Promise.reject(this.dead);
+    if (this.mode === "active" && !this.isPresence) return this.activate();
+    // Passive/presence: "synced" = the subscription reached an open socket.
+    if (this.subEverSent) return Promise.resolve();
+    return new Promise((resolve, reject) => {
+      this.subWaiters.push({ resolve, reject });
+    });
+  }
+
+  /**
+   * Upgrade to a real y-protocol participant and resolve once the doc holds
+   * the server state (first SyncStep2). Idempotent; rejects with
+   * {@link CollabSubRejectedError} if the gateway killed the channel.
+   */
+  activate(): Promise<void> {
+    if (this.dead) return Promise.reject(this.dead);
+    if (this.isPresence) return this.whenSynced();
+    if (this.synced) return Promise.resolve();
+    const wasPassive = this.mode === "passive";
+    this.mode = "active";
+    if (this.conn.isOpen()) {
+      if (wasPassive) this.conn.sendControl({ t: "act", ch: this.ch });
+      this.beginSync();
+    }
+    return new Promise((resolve, reject) => {
+      const waiter = {
+        resolve: () => {
+          clearTimeout(timer);
+          resolve();
+        },
+        reject: (e: unknown) => {
+          clearTimeout(timer);
+          reject(e);
+        },
+      };
+      // Bounded: doSwitch serializes on this — a dead socket must surface as
+      // a retryable failure, not a hung queue.
+      const timer = setTimeout(() => {
+        const i = this.syncWaiters.indexOf(waiter);
+        if (i >= 0) this.syncWaiters.splice(i, 1);
+        reject(
+          new Error(
+            `gateway activate for ${this.docPath} exceeded ${ACTIVATE_TIMEOUT_MS}ms`,
+          ),
+        );
+      }, ACTIVATE_TIMEOUT_MS);
+      this.syncWaiters.push(waiter);
+    });
+  }
+
+  /** `touched` hints (doc changed while passive) — sheet dirty flag. */
+  onTouched(cb: () => void): void {
+    this.touchedCbs.push(cb);
+  }
+
+  destroy(): void {
+    if (this.destroyed) return;
+    this.destroyed = true;
+    // Clean departure: the removal frame reaches peers while the socket is
+    // still up (the gateway's synthesized tombstone is only the crash path).
+    try {
+      removeAwarenessStates(this.awareness, [this.doc.clientID], "destroy");
+    } catch {
+      /* awareness already torn down */
+    }
+    this.awareness.off("update", this.onAwarenessUpdate);
+    if (!this.isPresence) this.doc.off("update", this.onDocUpdate);
+    this.awareness.destroy();
+    this.conn.unregister(this.ch);
+    const err = this.dead ?? new Error("gateway facade destroyed");
+    for (const w of this.syncWaiters.splice(0)) w.reject(err);
+    for (const w of this.subWaiters.splice(0)) w.reject(err);
+  }
+
+  // --- wire (called by GatewayConnection) -----------------------------------
+
+  subMsg(): GatewayClientMsg {
+    return { t: "sub", ch: this.ch, doc: this.docPath, mode: this.mode };
+  }
+
+  handleSocketOpen(): void {
+    if (this.destroyed || this.dead) return;
+    this.subEverSent = true;
+    for (const w of this.subWaiters.splice(0)) w.resolve();
+    // (Re)announce presence: a query for peers' states + our own, if any.
+    this.sendQueryAwareness();
+    if (this.awareness.getLocalState() !== null) this.publishLocalAwareness();
+    if (this.mode === "active" && !this.isPresence) this.beginSync();
+  }
+
+  handleSocketDown(): void {
+    // Same as y-websocket: peers' awareness is stale the moment the socket
+    // drops; local state survives and re-publishes on reconnect.
+    const remote = [...this.awareness.getStates().keys()].filter(
+      (id) => id !== this.doc.clientID,
+    );
+    if (remote.length > 0) {
+      removeAwarenessStates(this.awareness, remote, "connection closed");
+    }
+    this.synced = false;
+  }
+
+  handleControl(msg: GatewayServerMsg): void {
+    if (this.destroyed) return;
+    if (msg.t === "suberr") {
+      this.dead = new CollabSubRejectedError(
+        this.docPath,
+        msg.status,
+        msg.message,
+      );
+      cwarn(`[gateway] ${this.dead.message}`);
+      for (const w of this.syncWaiters.splice(0)) w.reject(this.dead);
+      for (const w of this.subWaiters.splice(0)) w.reject(this.dead);
+      return;
+    }
+    if (msg.t === "resync") {
+      // The doc's relay (re)connected server-side — our Step1 pulls the news.
+      if (this.mode === "active" && !this.isPresence) this.sendSyncStep1();
+      return;
+    }
+    // touched
+    for (const cb of this.touchedCbs) cb();
+  }
+
+  handleFrame(frame: Uint8Array): void {
+    if (this.destroyed || frame.length === 0) return;
+    try {
+      this.handleFrameInner(frame);
+    } catch (err) {
+      // A malformed frame must never take down the shared socket's handler.
+      cwarn(`[gateway] dropped undecodable frame for ${this.docPath}`, err);
+    }
+  }
+
+  private handleFrameInner(frame: Uint8Array): void {
+    const decoder = decoding.createDecoder(frame.slice());
+    const type = decoding.readVarUint(decoder);
+    if (type === MESSAGE_SYNC) {
+      if (this.isPresence) return;
+      const encoder = encoding.createEncoder();
+      encoding.writeVarUint(encoder, MESSAGE_SYNC);
+      const messageType = syncProtocol.readSyncMessage(
+        decoder,
+        encoder,
+        this.doc,
+        this,
+      );
+      if (encoding.length(encoder) > 1) {
+        this.send(encoding.toUint8Array(encoder));
+      }
+      if (messageType === syncProtocol.messageYjsSyncStep2 && !this.synced) {
+        this.synced = true;
+        for (const w of this.syncWaiters.splice(0)) w.resolve();
+      }
+      return;
+    }
+    if (type === MESSAGE_AWARENESS) {
+      applyAwarenessUpdate(
+        this.awareness,
+        decoding.readVarUint8Array(decoder),
+        "gateway-remote",
+      );
+      return;
+    }
+    if (type === MESSAGE_QUERY_AWARENESS) {
+      // A peer asks who is here — answer with our own state only.
+      if (this.awareness.getLocalState() !== null) this.publishLocalAwareness();
+    }
+  }
+
+  // --- internals ------------------------------------------------------------
+
+  private send(frame: Uint8Array): void {
+    this.conn.sendFrame(this.ch, frame);
+  }
+
+  private beginSync(): void {
+    this.sendSyncStep1();
+  }
+
+  private sendSyncStep1(): void {
+    const encoder = encoding.createEncoder();
+    encoding.writeVarUint(encoder, MESSAGE_SYNC);
+    syncProtocol.writeSyncStep1(encoder, this.doc);
+    this.send(encoding.toUint8Array(encoder));
+  }
+
+  private sendQueryAwareness(): void {
+    const encoder = encoding.createEncoder();
+    encoding.writeVarUint(encoder, MESSAGE_QUERY_AWARENESS);
+    this.send(encoding.toUint8Array(encoder));
+  }
+
+  private publishLocalAwareness(): void {
+    const encoder = encoding.createEncoder();
+    encoding.writeVarUint(encoder, MESSAGE_AWARENESS);
+    encoding.writeVarUint8Array(
+      encoder,
+      encodeAwarenessUpdate(this.awareness, [this.doc.clientID]),
+    );
+    this.send(encoding.toUint8Array(encoder));
+  }
+
+  private readonly onDocUpdate = (update: Uint8Array, origin: unknown): void => {
+    if (origin === this) return; // our own readSyncMessage apply
+    if (this.mode === "passive") {
+      // A local write into a passive doc (activate-before-write slipped) —
+      // auto-activate so the edit is neither dropped nor pushed unsynced.
+      void this.activate().catch(() => {
+        /* surfaced via the activate caller/suberr path */
+      });
+    }
+    const encoder = encoding.createEncoder();
+    encoding.writeVarUint(encoder, MESSAGE_SYNC);
+    syncProtocol.writeUpdate(encoder, update);
+    this.send(encoding.toUint8Array(encoder));
+  };
+
+  private readonly onAwarenessUpdate = (
+    {
+      added,
+      updated,
+      removed,
+    }: { added: number[]; updated: number[]; removed: number[] },
+    origin: unknown,
+  ): void => {
+    if (origin === "gateway-remote") return; // no echo loops
+    const changed = added.concat(updated).concat(removed);
+    if (changed.length === 0) return;
+    const encoder = encoding.createEncoder();
+    encoding.writeVarUint(encoder, MESSAGE_AWARENESS);
+    encoding.writeVarUint8Array(
+      encoder,
+      encodeAwarenessUpdate(this.awareness, changed),
+    );
+    this.send(encoding.toUint8Array(encoder));
+  };
+}
diff --git a/web/standalone/src/wasm/collab/index.ts b/web/standalone/src/wasm/collab/index.ts
index 815e534..847aba6 100644
--- a/web/standalone/src/wasm/collab/index.ts
+++ b/web/standalone/src/wasm/collab/index.ts
@@ -155,6 +155,11 @@ export async function connectKicadDoc(opts: {
   signal?: AbortSignal;
   /** Whole-path deadline; defaults to 30s. */
   timeoutMs?: number;
+  /** Gateway transport (load-path-rework 0003): register interest without
+   *  demanding doc state — the warm pool's parked sheets. `whenSynced` then
+   *  resolves on subscription, and `provider.activate?.()` is the real sync
+   *  barrier before any bind/read/write of the doc. */
+  passive?: boolean;
 }): Promise<KicadDocSession> {
   const timeoutMs = opts.timeoutMs ?? CONNECT_TIMEOUT_MS;
   // An already-aborted owner never gets a session — even one that could
@@ -186,6 +191,7 @@ export async function connectKicadDoc(opts: {
 
   const providerPromise = connectProvider(doc, opts.provider, {
     room: opts.room,
+    passive: opts.passive,
   });
   providerPromise.catch(() => {}); // may lose the race and reject later
 
diff --git a/web/standalone/src/wasm/collab/provider.ts b/web/standalone/src/wasm/collab/provider.ts
index 364b4f3..9b05a89 100644
--- a/web/standalone/src/wasm/collab/provider.ts
+++ b/web/standalone/src/wasm/collab/provider.ts
@@ -1,5 +1,6 @@
 import type * as Y from "yjs";
 import { Awareness } from "y-protocols/awareness";
+import { parseCollabRoomId } from "@pcbjam/shared";
 import { connectBroadcastChannel } from "./broadcast-transport";
 import { connectAwarenessBroadcast } from "./awareness-bc";
 
@@ -27,6 +28,18 @@ export interface YjsProvider {
    * sibling channel. Absent for `none` — presence UI then renders nothing.
    */
   awareness?: Awareness;
+  /**
+   * Gateway transport only (load-path-rework 0003): upgrade a `passive`
+   * subscription to a real y-protocol participant and resolve once the doc
+   * holds the server state. Providers without it are always fully synced
+   * after `whenSynced` — callers treat absence as an already-synced no-op.
+   */
+  activate?(): Promise<void>;
+  /**
+   * Gateway transport only: the doc changed server-side while this
+   * subscription was passive (the sheet manager's parked-dirty flag).
+   */
+  onTouched?(cb: () => void): void;
 }
 
 export type ProviderKind =
@@ -154,17 +167,38 @@ async function hocuspocusProvider(
 /**
  * Connect a Y.Doc to the env-selected provider. Network libraries are imported
  * lazily so a deployment only ships the one it uses.
+ *
+ * `passive` (gateway transport only): register interest without demanding doc
+ * state — the sheet manager's warm pool. Other providers ignore it and stay
+ * fully synced, a harmless superset.
  */
 export async function connectProvider(
   doc: Y.Doc,
   config: ProviderConfig,
-  opts: { room: string },
+  opts: { room: string; passive?: boolean },
 ): Promise<YjsProvider> {
   switch (config.kind) {
     case "broadcastchannel":
       return broadcastChannelProvider(doc, opts.room, config.settleMs ?? 300);
-    case "partykit":
+    case "partykit": {
+      // Load-path-rework 0003: board/presence rooms ride the per-project
+      // gateway socket (one ws per project instead of one per room). The
+      // legacy per-room dial stays only for room ids that don't parse — an
+      // operator/debug shape the gateway can't address.
+      const parsed = parseCollabRoomId(opts.room);
+      if (parsed) {
+        const { GatewayDocFacade } = await import("./gateway");
+        return new GatewayDocFacade(doc, {
+          endpoint: requireEndpoint(config),
+          scopeId: parsed.scopeId,
+          projectId: parsed.projectId,
+          docPath: parsed.docPath,
+          token: config.params?.token,
+          passive: opts.passive,
+        });
+      }
       return partyKitProvider(doc, requireEndpoint(config), opts.room, config.params);
+    }
     case "hocuspocus":
       return hocuspocusProvider(doc, requireEndpoint(config), opts.room, config.params);
     case "none":
diff --git a/web/standalone/src/wasm/collab/sheet-manager.ts b/web/standalone/src/wasm/collab/sheet-manager.ts
index 8991c31..27d6581 100644
--- a/web/standalone/src/wasm/collab/sheet-manager.ts
+++ b/web/standalone/src/wasm/collab/sheet-manager.ts
@@ -175,7 +175,10 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
     }
   }
 
-  async function ensureRoom(sheetPath: string): Promise<Room> {
+  async function ensureRoom(
+    sheetPath: string,
+    connectOpts?: { passive?: boolean },
+  ): Promise<Room> {
     const existing = rooms.get(sheetPath);
     if (existing) return existing;
     const inflight = connecting.get(sheetPath);
@@ -185,6 +188,7 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
       const session = await connectKicadDoc({
         provider,
         room: collabRoomId(scopeId, projectId, sheetPath),
+        passive: connectOpts?.passive,
       });
       if (destroyed) {
         // The manager died while this connect was in flight (unmount during
@@ -203,6 +207,12 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
         editorMatchesDoc: false,
         dirty: false,
       };
+      // Gateway transport: a `touched` hint means the doc changed server-side
+      // while this subscription was passive — same catch-up contract as the
+      // parked-update watch.
+      session.provider.onTouched?.(() => {
+        room.dirty = true;
+      });
       rooms.set(sheetPath, room);
       log(`[sheet] warm room connected: ${sheetPath}`);
       // A room warmed after the first bind starts parked — give it a skeleton
@@ -261,6 +271,12 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
 
     const room = await ensureRoom(sheetPath);
     if (destroyed) return;
+    // Gateway transport: a passively-warmed sheet holds no doc state yet —
+    // activate() is the real sync barrier (no-op for other providers and on
+    // revisits). Runs BEFORE the watch detaches so catch-up updates still
+    // mark `dirty` for the adopt decision below.
+    await room.session.provider.activate?.();
+    if (destroyed) return;
 
     // Activating: stop tracking parked updates and bind the (warm) doc to the editor.
     room.detachWatch?.();
@@ -316,8 +332,11 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
         // Terminal version skew (findings C-5): retrying cannot make an
         // incompatible doc encoding bindable. No retry timer — rethrow so the
         // caller surfaces it (boot converts it into the user-visible version
-        // error instead of reporting "connected" with no binding).
-        if ((err as { name?: string } | null)?.name === "SexprVersionError") {
+        // error instead of reporting "connected" with no binding). A gateway
+        // subscription rejection (invalid-file 409 — load-path-rework 0003)
+        // is terminal the same way: the flag only clears on a new upload.
+        const name = (err as { name?: string } | null)?.name;
+        if (name === "SexprVersionError" || name === "CollabSubRejectedError") {
           throw err;
         }
         // Still the sheet the editor shows and not yet bound → retry with backoff,
@@ -352,21 +371,36 @@ export function createSheetCollabManager(opts: SheetManagerOptions): SheetCollab
   function syncLayoutFromSave(sheetPath: string, fileText: string): void {
     const room = rooms.get(sheetPath);
     if (!room) return; // not a collab sheet (or still onboarding) — nothing to sync
-    try {
-      // Writing to a PARKED room's doc marks it dirty via startWatch — fine:
-      // the diff-on-rebind adopt makes the catch-up cost the real delta only.
-      if (syncLayoutToY(fileToDoc(fileText), room.doc, "layout-save")) {
-        clog(`[sheet] layout save-sync: ${sheetPath} updated`);
+    const write = (): void => {
+      try {
+        // Writing to a PARKED room's doc marks it dirty via startWatch — fine:
+        // the diff-on-rebind adopt makes the catch-up cost the real delta only.
+        if (syncLayoutToY(fileToDoc(fileText), room.doc, "layout-save")) {
+          clog(`[sheet] layout save-sync: ${sheetPath} updated`);
+        }
+      } catch (err) {
+        cwarn(`[sheet] layout save-sync failed for ${sheetPath}`, err);
       }
-    } catch (err) {
-      cwarn(`[sheet] layout save-sync failed for ${sheetPath}`, err);
+    };
+    // Gateway transport: a passively-warmed doc is empty — writing into it
+    // would push partial state. A save IS write demand, so sync first.
+    const activate = room.session.provider.activate?.();
+    if (activate) {
+      void activate.then(write).catch((err) => {
+        cwarn(`[sheet] layout save-sync activate failed for ${sheetPath}`, err);
+      });
+    } else {
+      write();
     }
   }
 
   async function connectAll(sheetPaths: string[]): Promise<void> {
     await Promise.all(
       sheetPaths.map((p) =>
-        ensureRoom(p).catch((err) => {
+        // Passive on the gateway (load-path-rework 0003): warm-all becomes
+        // pure registration — awareness + touched hints flow, but no
+        // BoardRoom wakes for a sheet nobody opens or edits.
+        ensureRoom(p, { passive: true }).catch((err) => {
           cwarn(`[sheet] failed to warm ${p}`, err);
           return null;
         }),