WF-0128: File watcher and external move detection #165

Hill

Jim detects host path changes proactively enough to update frontier posture, while the save/open contract remains correct without depending on watcher timing.

Why

PR #160 made host preflight safe even without watchers. Watchers are an optimization and UX improvement: they should notify Jim earlier when files are changed, created, deleted, or moved externally.

Initial acceptance shape

• Host file create/change/delete notifications can refresh frontier posture.
• External move/rename detection is scoped and honest about path identity limits.
• Watcher failures degrade to save/focus/refresh-time observation.
• No save correctness depends solely on watcher delivery.
• Tests cover stale/missed watcher events and path-kind changes.

Non-goals

• Symlink/hardlink/case-insensitive identity semantics in the first slice.
• Multi-file transaction reconciliation.

Related

• Split out of WF-0123: Unmaterialized file buffers and external edit frontier #159 after PR Harden Echo text projection and file materialization semantics #160.
• Complements External Edit intake.