feat(pcbnew): Yjs collab bridge — snapshot/diff emit + BOARD_COMMIT apply (move/delete/track-add), verified two-tab in real app

pcbnew's half of the unified Yjs collab bridge (4th tool; yjs-bridge commit 4),
a near-verbatim port of the eeschema design. Root-repo only — kicad/wxwidgets
submodules untouched.

- wasm/bindings/pcbnew_embind.cpp: BOARD_LISTENER trigger + post-settle snapshot
  diff emit; BOARD_COMMIT apply inside a CallAfter + COROUTINE fiber (so a new
  item's GAL view->Add dispatches correctly). Move/delete sync for any top-level
  item by uuid; native PCB_TRACK add. Footprint/via/zone add deferred.
- WasmTool.tsx: add pcbnew to COLLAB_TOOLS.
- tests/apps/kicad/pcbnew-collab.html: seeded (wizard-free) harness, leaving
  pcbnew.html untouched for its wizard test.
- tests/kicad/pcbnew-collab.spec.ts: snapshot + apply(move/remove/add) — 2 pass,
  two-tab skipped headless.

Verified two-tab in the real web app: footprint move applies + syncs A->B.

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

web/apps/frontend/src/components/WasmTool.tsx

@@ -9,7 +9,7 @@ import type { CollabWindow } from "@/wasm/collab";
 import { clog, cwarn } from "@/wasm/collab/debug";
 
 // Tools with a working collab bridge (kicadCollabSnapshot/Apply embind exports).
-const COLLAB_TOOLS = new Set<Tool>(["pl_editor", "eeschema"]);
+const COLLAB_TOOLS = new Set<Tool>(["pl_editor", "eeschema", "pcbnew"]);
 
 /**
  * Opt-in collaborative editing (features/yjs-bridge). Enabled when the URL carries