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.
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