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>

.changeset/engine-singleton-lock.md

@@ -0,0 +1,20 @@
+---
+"@fusion/engine": minor
+---
+
+feat(engine): guard one engine per project per machine
+
+Adds a per-machine singleton lock that engages before each engine
+starts, preventing two `fn` dashboard processes from running engines
+for the same project on the same host (a scenario that previously
+caused worktree corruption and task-state races for in-process
+projects).
+
+The guard combines two independent checks:
+- A `proper-lockfile`-backed file at `<project>/.fusion/engine.lock`
+  with stale-lock recovery.
+- A loopback listener (UDS on POSIX, named pipe on Windows) on a
+  hashed per-project address.
+
+Failures throw `EngineAlreadyRunningError`; both guards are released
+on `stopAll()` / `pauseProject()`.