Add Chromium-only Blender WebEngine parity work

docs/status/W-012.md

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+# W-012 Status
+
+status: done
+task: 建立 EngineClient 和 Worker 通信
+
+## Evidence
+
+- Client: `web/app/src/engine-client/EngineClient.ts`
+- Worker: `web/app/src/workers/engine.worker.ts`
+- Protocol: `web/protocol/engine.ts`, `web/protocol/scene-ir.ts`
+- The browser status changes to `Mock Engine: r0` after init and snapshot.
+- Screenshot: `tests/golden/W-012/mock-worker-1440x900.png`
+
+## Verification
+
+```text
+npm run typecheck  # passed
+npm run lint       # passed
+npm run build      # passed
+Chrome headless --virtual-time-budget=4000  # Mock Engine: r0
+```