Add transcript search and per-activity detail controls (#46734)

## What changed

- Add incremental, case-insensitive literal search across transcript details, loading older history as needed. Use `F3` to start Find, or `/` in the transcript pager, and navigate matches in either direction. Closing Find restores the previous presentation and reading position.
- Add clickable activity detail controls and `F4` keyboard focus for inspecting individual activity groups in the owned transcript. Preserve expansion state as live activity becomes history.
- Expose configurable `global.find_transcript`, `global.focus_activity`, and `pager.find` bindings under `tui.keymap`. Let new defaults yield to existing custom bindings and show available completions for pending chords.

## Testing

Add regression coverage for Unicode matching, bounded scanning, paginated and changing history, selection during search, composer input isolation, activity controls, and custom shortcut conflicts.

GitOrigin-RevId: 5bcbcdfbca7c5c296c635ec9b079ccd7a908e336

codex-rs/tui/src/keymap_setup.rs

@@ -1493,7 +1493,7 @@ mod tests {
         let mut view = build_keymap_debug_view(&RuntimeKeymap::defaults(), &TuiKeymap::default());
         view.show_delayed_hint_for_test();
 
-        view.handle_key_event(KeyEvent::new(KeyCode::Char('o'), KeyModifiers::CONTROL));
+        view.handle_key_event(KeyEvent::new(KeyCode::F(8), KeyModifiers::NONE));
 
         let rendered = render_debug(&view, /*width*/ 100);
         assert!(!rendered.contains("Still waiting?"));