The previous code called std::process::exit(1) on file load failure,
bypassing all terminal restoration (disable_raw_mode, LeaveAlternateScreen,
show_cursor). This left the user's shell in a broken state with no echo
and no visible cursor.
Introduce TerminalGuard with Drop-based cleanup that fires on every exit
path: normal return, ? error propagation, and panic unwind. The guard
also handles partial initialization rollback (e.g. raw mode enabled but
alternate screen fails). Remove all process::exit calls from the guarded
scope.
Replace synchronous file loading with AppLoadingState state machine (Empty/Loading/Ready/Error) for instant interactivity. Add ViewportCache for on-demand viewport computation, replacing global wrap/level caches. Integrate background indexer polling and file watcher events into the TUI event loop. Add loading UI with progress percentage, estimated line numbers with ~ prefix, and error state display. Eliminate all O(N) linear scans using VisualHeightIndex binary search.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
Add AppMode enum (Normal/Settings), level_cache field populated in recompute_wrap_cache via detect_level(), color_config loaded from TOML in main.rs. Refactor handle_key for mode dispatch with S key to enter settings.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
