Structured FileState dogfood key

[0xLeif]

Part of GOAL.md workstream B.

Add one Codable model key (fixed schema, e.g. profile with a couple of typed fields) persisted through FileState, exercising persistence, paths, Codable encoding, and watch on a structured value. No dynamic schemas (v1 non-goal).

Acceptance criteria

• New DemoKey backed by FileState storing a Codable struct
• set/get round-trip with per-field input and validation errors
• watch observes cross-process writes (direct disk read path, like note)
• dump includes the structured value as nested JSON
• specs updated in the same change

[0xLeif]

Shipped in PR #2 (aps 0.2.0, merge commit 62198af). Verified in review: gates green, behavior confirmed locally.