feat(pl_editor): Yjs collaborative bridge — differ/apply + generic reconciler (yjs-bridge commit 2)

Bidirectional bridge between pl_editor's DS_DATA_MODEL and a Yjs doc, two same-origin
tabs syncing over BroadcastChannel. Full architecture in features/yjs-bridge/0001-0002.

C++ (wasm layer, wasm/bindings/pl_editor_embind.cpp — public DS_DATA_MODEL API only,
zero added fork divergence beyond the OnModify hook):
- snapshot-differ ChangeSource: diff model vs last-emitted snapshot on OnModify,
  emit per-item delta JSON via EM_ASM window.kicadCollab.onDelta
- kicadCollabApply(json): apply remote delta by uuid — scalars (text/segment/rect)
  by field, polygon/bitmap via SetPageLayout-append blob; reseed snapshot + HardRedraw
- kicadCollabSnapshot() (seed/baseline), s_applyingRemote echo guard, and a
  kicadCollabTestAddText() PoC local-edit hook
- wire format: {added:[item],changed:[item],removed:[uuid]}, item = {id,type,...fields}

JS (web/apps/frontend/src/wasm/collab/, generic + schema-agnostic):
- reconciler: uuid-keyed Y.Map of per-item Y.Map; down = onDelta→Y, up = observe→apply,
  origin-tagged echo suppression; seed-once join adopts the doc authoritatively
- broadcast-transport: minimal BroadcastChannel Yjs provider (query/state catch-up)
- WasmTool wiring behind ?collab=1 (pl_editor only); gated debug logging

Tests: tests/kicad/pl_editor-collab.spec.ts — single-page C++ contract (snapshot/apply
changed+removed+added/echo-suppression) + two-tab BroadcastChannel A<->B propagation.
Reconciler+yjs bundled via esbuild (tests/collab/build.mjs, npm run build:collab).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/package.json

@@ -7,6 +7,7 @@
     "test:ui": "playwright test --ui",
     "test:headed": "playwright test --headed",
     "build-wasm": "cd apps && make -f Makefile.wasm",
+    "build:collab": "node collab/build.mjs",
     "serve": "npx serve apps -p 8080 -c ../serve.json",
     "setup:kicad": "./scripts/setup-kicad-wasm.sh",
     "test:web": "npm run setup:kicad && playwright test --config=playwright-web.config.ts --project=firefox",
@@ -39,7 +40,9 @@
   "devDependencies": {
     "@playwright/test": "^1.40.0",
     "@types/node": "^24.10.1",
+    "esbuild": "^0.28.0",
     "serve": "^14.2.0",
-    "typescript": "^5.9.3"
+    "typescript": "^5.9.3",
+    "yjs": "^13.6.31"
   }
 }