Support two-stroke TUI key chords (#36511)

## What changed

- Accept two-stroke bindings such as `ctrl-x ctrl-s` in TUI keymap configuration while preserving arrays as alternative bindings.
- Route chords through the active TUI context, show pending and configured chord hints, and cancel pending chords on `esc`, context changes, or a one-second timeout.
- Extend `/keymap` to capture, add, replace, and display key chords.
- Reject chords that conflict with active single-key bindings, overlapping-context chords, or reserved terminal shortcuts.

## Testing

- Add coverage for chord parsing, validation, matching, timeout and cancellation behavior, context dispatch, key capture, and rendered shortcut hints.

GitOrigin-RevId: dfdede03f518d4936ad8621ebf5537026f2b282e

codex-rs/tui/src/app_event.rs

@@ -176,6 +176,13 @@ pub(crate) enum KeymapEditIntent {
     ReplaceOne { old_key: String },
 }
 
+/// Number of key strokes recorded by one `/keymap` capture.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum KeymapCaptureMode {
+    SingleKey,
+    Chord,
+}
+
 #[allow(clippy::large_enum_variant)]
 #[derive(Debug)]
 pub(crate) enum AppEvent {
@@ -1116,6 +1123,7 @@ pub(crate) enum AppEvent {
         context: String,
         action: String,
         intent: KeymapEditIntent,
+        capture_mode: KeymapCaptureMode,
     },
 
     /// Open the keymap keypress inspector.