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/input.rs

@@ -7,6 +7,83 @@ use super::*;
 use crate::app_backtrack::SIDE_EDIT_PREVIOUS_UNAVAILABLE_MESSAGE;
 
 impl App {
+    pub(super) fn route_key_chord_event(
+        &mut self,
+        tui: &mut tui::Tui,
+        key_event: KeyEvent,
+    ) -> Option<KeyEvent> {
+        let contexts = self.active_keymap_contexts();
+        let was_pending = self.key_chord_matcher.is_pending();
+        match self.key_chord_matcher.advance(
+            key_event,
+            &self.keymap.chords,
+            contexts,
+            tokio::time::Instant::now(),
+        ) {
+            crate::keymap::KeyChordMatch::PassThrough => {
+                if was_pending && !self.key_chord_matcher.is_pending() {
+                    self.chat_widget.set_footer_hint_override(/*items*/ None);
+                }
+                Some(key_event)
+            }
+            crate::keymap::KeyChordMatch::Pending(prefix) => {
+                if self.backtrack.primed {
+                    self.reset_backtrack_state();
+                }
+                self.chat_widget.set_footer_hint_override(Some(vec![
+                    (
+                        format!("{} …", prefix.display_label()),
+                        "waiting for next key".to_string(),
+                    ),
+                    ("esc".to_string(), "cancel".to_string()),
+                ]));
+                tui.frame_requester()
+                    .schedule_frame_in(crate::keymap::KEY_CHORD_TIMEOUT);
+                None
+            }
+            crate::keymap::KeyChordMatch::Completed(dispatch_event) => {
+                self.chat_widget.set_footer_hint_override(/*items*/ None);
+                Some(dispatch_event)
+            }
+            crate::keymap::KeyChordMatch::Cancelled => {
+                self.chat_widget.set_footer_hint_override(/*items*/ None);
+                None
+            }
+            crate::keymap::KeyChordMatch::Ignored => None,
+        }
+    }
+
+    pub(super) fn expire_pending_key_chord(&mut self) {
+        let contexts = self.active_keymap_contexts();
+        if self
+            .key_chord_matcher
+            .expire(contexts, tokio::time::Instant::now())
+        {
+            self.chat_widget.set_footer_hint_override(/*items*/ None);
+        }
+    }
+
+    pub(super) fn cancel_pending_key_chord(&mut self) {
+        if self.key_chord_matcher.cancel() {
+            self.chat_widget.set_footer_hint_override(/*items*/ None);
+        }
+    }
+
+    fn active_keymap_contexts(&self) -> crate::keymap::KeymapContextSet {
+        if self.overlay.is_some() {
+            return crate::keymap::KeymapContextSet::new(crate::keymap::KeymapContext::Pager);
+        }
+
+        let contexts = self.chat_widget.keymap_contexts();
+        if self.chat_widget.no_modal_or_popup_active() {
+            contexts
+                .with(crate::keymap::KeymapContext::Global)
+                .with(crate::keymap::KeymapContext::Chat)
+        } else {
+            contexts
+        }
+    }
+
     pub(super) async fn launch_external_editor(&mut self, tui: &mut tui::Tui) {
         let editor_cmd = match external_editor::resolve_editor_command() {
             Ok(cmd) => cmd,