Why

This stack moves codex-tui away from the core protocol event surface and toward app-server API shapes plus TUI-owned local models. This first PR sets up the lower-risk foundation: it introduces the local model surface and extracts app-server event routing into focused TUI modules while preserving the existing behavior for the larger migration in PR2.

This PR is part 1 of a 2-PR stack:

1. Add TUI-owned replacement models and extract app-server event routing.
2. Move the active TUI flow to app-server notifications and delete obsolete adapter code.

What changed

• Added TUI-owned approval, diff, session state, session resume, token usage, and user-message models.
• Added app/app_server_event_targets.rs and app/app_server_events.rs to hold app-server event targeting and dispatch logic outside app.rs.
• Updated app/status tests to use the local model layer and added focused routing coverage.
• Boxed a few large async TUI test futures so this base layer remains checkable without overflowing the default test stack.

Verification

• cargo check -p codex-tui --tests