Add behavioral coverage for runtime UI and platform FFI surface

[TheHypnoo]

Follow-up to #694.

Goal

Convert the runtime UI and platform FFI inventory into behavioral coverage that can run on the appropriate host or target.

Scope

Start from test-files/test_ffi_surface_runtime_ui.ts and move coverage entries into target-appropriate tests.

Suggested areas:

• TUI node creation, styling, layout, input, hooks, and render behavior
• Geisterhand registry and action queue behavior
• i18n formatting and interpolation helpers
• media playback state helpers where testable without platform media services
• platform callback shims for ArkTS, iOS, watchOS, and related UI targets

Acceptance criteria

• Host-independent TUI/i18n behavior gets normal deterministic tests.
• Platform-specific behavior is covered by simulator/emulator smoke tests or target-gated scripts.
• Converted entries are removed from test_ffi_surface_runtime_ui.ts by running ./test-coverage/regen_ts_surface_inventory.py.
• Relevant parity, doc-test, simulator, or UI smoke scripts are documented in the test plan.
• ./test-coverage/audit.sh --markdown still reports full TypeScript FFI surface coverage.