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/src/__tests__/project-engine-manager.test.ts

@@ -1,5 +1,16 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 
+// Stub out the per-machine singleton lock so tests with fake working dirs
+// (e.g. /mapped/...) don't try to mkdir or bind real sockets.
+vi.mock("../engine-singleton-lock.js", () => ({
+  acquireEngineSingleton: vi.fn().mockResolvedValue({
+    release: vi.fn().mockResolvedValue(undefined),
+    socketPath: "/tmp/test.sock",
+    lockFilePath: "/tmp/test.lock",
+  }),
+  EngineAlreadyRunningError: class EngineAlreadyRunningError extends Error {},
+}));
+
 // Mock ProjectEngine before importing the manager
 vi.mock("../project-engine.js", () => {
   return {