Add file history to the viewer — the last remaining MC viewer gap.
open_next/open_prev push the current path to a file_history vec.
Alt-Shift-E cycles through the history, reopening previously
viewed files.
- Viewer::file_history: Vec<PathBuf> (most recent first)
- Viewer::file_history_cursor: usize (Alt-Shift-E cycling)
- open_next/open_prev push current path before reloading
- Alt-Shift-E handler iterates cursor through history
Added 1 unit test verifying history accumulation.
Tests: 1432 pass (was 1431), zero warnings.
vasilito
703d6c8cea