feat(standalone): 8-way parallel lib presync + stable load counters

Library warm-up and the per-item fallback fat-load now run through a
shared asyncMap helper (counting semaphore, 8 in flight): the CDN
presync's hand-rolled 6-worker pool is replaced and the previously
fully-serial listItems+getItemBody fallback is parallelized (order
preserved).

The load-screen sync line stops flashing: progress reports on lib
COMPLETION only (no "current lib" — with 8 in flight there isn't one),
the label is a fixed prefix ("Syncing symbol libraries — ") and the
counter is space-padded to the total's width in font-mono +
whitespace-pre, so 2→3 digit growth doesn't shift the text. Same
padding for the fat-load overlay's "x / y libraries" line.

Verified in demo mode against the live R2 CDN: 7-8 overlapping
manifest/bundle requests, 222 cold symbol libs in a few seconds.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgThWXtdvrYLK47EDFoGdq

web/standalone/src/components/WasmTool.tsx

@@ -619,8 +619,14 @@ export function WasmTool({
   // A library item currently being fetched (open/save), for a transient spinner.
   const [libBusy, setLibBusy] = React.useState<string | null>(null);
   // Load-screen pre-sync progress: warming the project's lib bundles into IDB in
-  // parallel with the wasm download. Null when idle/done.
-  const [libSync, setLibSync] = React.useState<string | null>(null);
+  // parallel with the wasm download. Null when idle/done. Counts only (no
+  // "current lib") — the fetches run several-at-a-time, so there is no single
+  // current one, and a fixed label keeps the line still while it ticks.
+  const [libSync, setLibSync] = React.useState<{
+    kind: string;
+    done: number;
+    total: number;
+  } | null>(null);
   // Last lib error (e.g. a backend 404 on open), shown as a dismissible toast.
   const [libError, setLibError] = React.useState<string | null>(null);
   // Eager whole-library idb→wasm load in flight (the ~tens-of-seconds fat-load on
@@ -876,11 +882,9 @@ export function WasmTool({
           void source
             .presync({
               kind: libKind,
-              onProgress: ({ done, total, current }) =>
+              onProgress: ({ done, total }) =>
                 setLibSync(
-                  done >= total
-                    ? null
-                    : `Syncing ${libKind}s — ${current} (${done}/${total})`,
+                  done >= total ? null : { kind: libKind, done, total },
                 ),
             })
             .then(() => setLibSync(null))
@@ -1095,7 +1099,9 @@ export function WasmTool({
               </p>
               <DownloadProgress progress={progress} />
               {libSync && (
-                <p className="font-mono text-xs text-emerald-300/90">{libSync}</p>
+                <p className="whitespace-pre font-mono text-xs text-emerald-300/90">
+                  {libSyncLabel(libSync)}
+                </p>
               )}
               <p className="font-mono text-xs text-white/40">
                 First load downloads the tool (large) — this can take a moment.
@@ -1142,9 +1148,14 @@ export function WasmTool({
                   }}
                 />
               </div>
-              <p className="mt-1 text-center font-mono text-[11px] text-white/50">
-                {Math.min(libLoading.done, libLoading.total)} / {libLoading.total}{" "}
-                libraries
+              {/* Space-pad `done` to `total`'s width so the centered line
+                  doesn't shift as the count gains a digit. */}
+              <p className="mt-1 whitespace-pre text-center font-mono text-[11px] text-white/50">
+                {String(Math.min(libLoading.done, libLoading.total)).padStart(
+                  String(libLoading.total).length,
+                  " ",
+                )}{" "}
+                / {libLoading.total} libraries
               </p>
             </div>
           )}
@@ -1189,8 +1200,9 @@ export function WasmTool({
       {/* Lib pre-sync still warming IDB after the editor opened (big set) — small
           unobtrusive indicator so the user knows browsing is still filling in. */}
       {ready && libSync && (
-        <div className="pointer-events-none absolute bottom-9 left-3 z-20 flex items-center gap-2 rounded bg-black/80 px-3 py-1.5 text-xs text-emerald-200">
-          <Loader2 className="animate-spin" size={14} /> {libSync}
+        <div className="pointer-events-none absolute bottom-9 left-3 z-20 flex items-center gap-2 rounded bg-black/80 px-3 py-1.5 font-mono text-xs text-emerald-200">
+          <Loader2 className="animate-spin" size={14} />{" "}
+          <span className="whitespace-pre">{libSyncLabel(libSync)}</span>
         </div>
       )}
 
@@ -1237,6 +1249,18 @@ export function WasmTool({
   );
 }
 
+/**
+ * Fixed-width lib pre-sync line, e.g. "Syncing symbol libraries —  42/208".
+ * The prefix is constant and `done` is space-padded to `total`'s digit count,
+ * so the text stays still while the counter ticks (render it in a font-mono +
+ * whitespace-pre element so the pad spaces hold their width).
+ */
+function libSyncLabel(s: { kind: string; done: number; total: number }): string {
+  const total = String(s.total);
+  const done = String(Math.min(s.done, s.total)).padStart(total.length, " ");
+  return `Syncing ${s.kind} libraries — ${done}/${total}`;
+}
+
 /**
  * WASM download progress for the boot overlay. A determinate bar when the server
  * sent a Content-Length the decoded stream agrees with; otherwise just MB so far

web/standalone/src/lib/async-map.ts

@@ -0,0 +1,58 @@
+/**
+ * A counting semaphore: `acquire()` resolves immediately while permits remain,
+ * otherwise queues (FIFO) until a `release()` hands the permit over.
+ */
+class Semaphore {
+  private waiters: (() => void)[] = [];
+
+  private permits: number;
+
+  constructor(max: number) {
+    this.permits = max;
+  }
+
+  async acquire(): Promise<void> {
+    if (this.permits > 0) {
+      this.permits--;
+      return Promise.resolve();
+    }
+    return new Promise((resolve) => {
+      this.waiters.push(resolve);
+    });
+  }
+
+  release(): void {
+    const next = this.waiters.shift();
+    if (next) next();
+    else this.permits++;
+  }
+}
+
+/**
+ * `Promise.all(array.map(callback))` with at most `concurrencyLimit` callbacks
+ * in flight at once. Results keep the input order. A callback rejection
+ * rejects the whole map (wrap the callback if per-item failures should be
+ * tolerated).
+ */
+export async function asyncMap<T, U>(
+  array: T[],
+  callback: (item: T, index: number, array: T[]) => Promise<U>,
+  concurrencyLimit: number,
+): Promise<U[]> {
+  const semaphore = new Semaphore(concurrencyLimit);
+  const results: U[] = [];
+
+  const wrappedCallback = async (item: T, index: number, srcArray: T[]) => {
+    await semaphore.acquire();
+    try {
+      const result = await callback(item, index, srcArray);
+      results[index] = result;
+    } finally {
+      semaphore.release();
+    }
+  };
+
+  await Promise.all(array.map((item, index) => wrappedCallback(item, index, array)));
+
+  return results;
+}

web/standalone/src/wasm/libs/cdn-source.ts

@@ -1,4 +1,5 @@
 import { SyncStack, type SyncStackOptions } from "@pcbjam/sync-client";
+import { asyncMap } from "@/lib/async-map";
 import type { LibInfo, LibItemInfo, LibsSource } from "./source";
 
 /**
@@ -122,7 +123,7 @@ export function cdnLibsSource(
       return (await stack.list()).map((e) => splitPath(e.path));
     },
     async presync(opts): Promise<void> {
-      const { kind, concurrency = 6, onProgress, signal } = opts ?? {};
+      const { kind, concurrency = 8, onProgress, signal } = opts ?? {};
       let m: CdnLibsManifest;
       try {
         m = await loadManifest();
@@ -136,25 +137,23 @@ export function cdnLibsSource(
       const total = libs.length;
       let done = 0;
       onProgress?.({ done, total, current: "" });
-      // Concurrency-limited pool: each openStack cold-fetches one bundle into IDB
+      // Concurrency-limited: each openStack cold-fetches one bundle into IDB
       // (warm ⇒ a cheap manifest diff). Tolerate per-lib failures so one bad lib
-      // doesn't abort the warm-up.
-      let idx = 0;
-      const worker = async (): Promise<void> => {
-        while (idx < libs.length) {
+      // doesn't abort the warm-up. Progress is reported on COMPLETION only —
+      // with several libs in flight there is no single "current" one.
+      await asyncMap(
+        libs,
+        async (lib) => {
           if (signal?.aborted) return;
-          const lib = libs[idx++]!;
-          onProgress?.({ done, total, current: lib.name });
           try {
             await openStack(lib.id);
           } catch {
             // best-effort: the lib still loads lazily on demand
           }
+          if (signal?.aborted) return;
           onProgress?.({ done: ++done, total, current: lib.name });
-        }
-      };
-      await Promise.all(
-        Array.from({ length: Math.min(concurrency, total) }, () => worker()),
+        },
+        concurrency,
       );
     },
     async getAllItems(

web/standalone/src/wasm/libs/source.ts

@@ -1,3 +1,4 @@
+import { asyncMap } from "@/lib/async-map";
 import { handleModel3dRequest } from "./models-bridge";
 import { libIdFromUri, libUri } from "./uri";
 
@@ -70,7 +71,7 @@ export interface LibsSource {
   presync?(opts?: {
     /** Limit to libs holding this item kind ("symbol" | "footprint"). */
     kind?: string;
-    /** Max concurrent bundle fetches (default 6). */
+    /** Max concurrent bundle fetches (default 8). */
     concurrency?: number;
     onProgress?: (p: LibPresyncProgress) => void;
     signal?: AbortSignal;
@@ -189,7 +190,8 @@ const sleep = (ms: number) => new Promise((r) => setTimeout(r, ms));
  * Slow-path "fat list" for sources without a bulk `getAllItems` (the example
  * backend / per-item remote): just listItems + one getItemBody each. Same N
  * round-trips as before — but it keeps the single WASM-side code path (the plugin
- * always asks for bodies once) working against every source.
+ * always asks for bodies once) working against every source. The per-item gets
+ * run 8 at a time (they're independent HTTP round-trips); order is preserved.
  */
 async function fallbackGetAllItems(
   source: LibsSource,
@@ -197,13 +199,17 @@ async function fallbackGetAllItems(
 ): Promise<Array<{ kind: string; name: string; body: Uint8Array }>> {
   const items = await source.listItems(libId);
   const enc = new TextEncoder();
-  const out: Array<{ kind: string; name: string; body: Uint8Array }> = [];
-  for (const it of items) {
-    const body = await source.getItemBody(libId, it.kind, it.name);
-    if (body != null)
-      out.push({ kind: it.kind, name: it.name, body: enc.encode(body) });
-  }
-  return out;
+  const bodies = await asyncMap(
+    items,
+    (it) => source.getItemBody(libId, it.kind, it.name).catch(() => null),
+    8,
+  );
+  return items.flatMap((it, i) => {
+    const body = bodies[i];
+    return body != null
+      ? [{ kind: it.kind, name: it.name, body: enc.encode(body) }]
+      : [];
+  });
 }
 
 /** Escape a string for a KiCad s-expr quoted token. */