Harden pre-identity browser daemon cleanup

[ElbertePlinio]

Parent: #17
Refs #18 and #28

Problem

During the one-second startup window before the owned browser supervisor identity is persisted, stopOwnedBrowserDaemon kills only the supervisor child and reports cleanup complete. A pathological /proc identity-read failure combined with a supervisor crash could leave same-process-group Chrome members alive while runtime data is removed.

The normal path is protected by verified process-group teardown, and the supervisor ordinarily stays alive until all group members exit. This is defensive hardening, not a current shipping blocker.

Checklist

• Confirm the owned process group has no live members before returning gone
• Add a regression test for a missing supervisor with surviving group members
• Preserve PID-reuse refusal and retryable cleanup semantics
• Run focused browser lifecycle tests and typecheck

No feature flag: cleanup hardening only; no user-facing rollout behavior.

Current status: Deferred from PR #28 review
Next: implement after the browser lifecycle slice lands.