Restore terminal on TUI error/panic path (#214)

The interactive client called `result?` before `cleanup_tui_runtime`,
so any error propagated out of the run loop skipped terminal restore and
left it in raw mode / alternate screen. Typed input then appeared
invisible after exit because echo/cooked mode were never restored.

Wrap the runtime teardown in a RAII `TuiRuntimeGuard` whose `Drop`
performs a full restore unless teardown was explicitly completed via
`finish`/`finish_for_run_result`. This guarantees restoration on error
and panic-unwind paths. Add unit tests covering the drop-restore and
finish-disarm behavior.