feat(engine): guard one engine per project per machine

Adds a per-machine singleton lock that engages in
ProjectEngineManager.createAndStart() before any engine subsystems
spin up. Two fn dashboard processes can no longer run engines for the
same project on the same host — previously they would share .fusion/
state and corrupt worktrees / task rows for in-process projects.

The guard combines two independent checks:
  - A proper-lockfile file at <project>/.fusion/engine.lock with
    stale-lock recovery (auto-released on process death).
  - A loopback listener on a hashed per-project address — UDS on
    POSIX, named pipe on Windows. Stale UDS files are probed and
    unlinked before a retry bind.

Failures raise EngineAlreadyRunningError. Both guards are released
from stopAll() and pauseProject(); a release on engine.start()
failure lets retries re-acquire cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/package.json

@@ -39,12 +39,14 @@
     "@fusion/pi-claude-cli": "workspace:*",
     "@mariozechner/pi-ai": "^0.73.0",
     "@mariozechner/pi-coding-agent": "^0.73.0",
-    "typebox": "^1.0.0",
-    "cron-parser": "^5.5.0"
+    "cron-parser": "^5.5.0",
+    "proper-lockfile": "^4.1.2",
+    "typebox": "^1.0.0"
   },
   "devDependencies": {
     "@modelcontextprotocol/sdk": "^1.0.0",
     "@types/node": "^25.5.0",
+    "@types/proper-lockfile": "^4.1.4",
     "@vitest/coverage-v8": "^3.1.0",
     "typescript": "^5.7.0",
     "vitest": "^3.1.0"