Paginate transcript history in the TUI (#36948)

## Why

Paginated threads should not require the TUI to load and render their entire
history when a session is resumed.

## What changed

- Hydrate a bounded initial history page for paginated resumes and forks,
  respecting the configured terminal reflow row budget.
- Load older transcript items in the background as users navigate upward, while
  preserving the viewport, live output, and backtrack selection.
- Reconcile review-mode prompts across page boundaries and keep restored token
  usage hidden until it can be attributed to a persisted turn.

## Testing

- Added coverage for paginated resume and fork hydration, older-page merging,
  transcript prepending, backtrack behavior, review prompt filtering, and token
  usage restoration.

GitOrigin-RevId: 42160b943a9f6a1ba0cfe4b195c24ddf410b747d

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -620,6 +620,7 @@ impl ChatComposer {
                 flash: None,
                 context_window_percent: None,
                 context_window_used_tokens: None,
+                context_window_pending: false,
                 collaboration_mode_indicator: None,
                 goal_status_indicator: None,
                 ide_context_active: false,
@@ -1353,10 +1354,14 @@ impl ChatComposer {
     }
 
     fn right_footer_line_with_context(&self) -> Line<'static> {
-        let mut line = context_window_line(
-            self.footer.context_window_percent,
-            self.footer.context_window_used_tokens,
-        );
+        let mut line = if self.footer.context_window_pending {
+            Line::default()
+        } else {
+            context_window_line(
+                self.footer.context_window_percent,
+                self.footer.context_window_used_tokens,
+            )
+        };
         if let Some(vim_mode) = self.vim_mode_indicator_span() {
             line.spans.push(" | ".dim());
             line.spans.push(vim_mode);
@@ -4218,6 +4223,10 @@ impl ChatComposer {
         self.footer.context_window_used_tokens = used_tokens;
     }
 
+    pub(crate) fn set_context_window_pending(&mut self, pending: bool) {
+        self.footer.context_window_pending = pending;
+    }
+
     pub(crate) fn set_esc_backtrack_hint(&mut self, show: bool) {
         self.footer.esc_backtrack_hint = show;
         if show {

codex-rs/tui/src/bottom_pane/chat_composer/footer_state.rs

@@ -23,6 +23,7 @@ pub(super) struct FooterState {
     pub(super) flash: Option<FooterFlash>,
     pub(super) context_window_percent: Option<i64>,
     pub(super) context_window_used_tokens: Option<i64>,
+    pub(super) context_window_pending: bool,
     pub(super) collaboration_mode_indicator: Option<CollaborationModeIndicator>,
     pub(super) goal_status_indicator: Option<GoalStatusIndicator>,
     pub(super) ide_context_active: bool,

codex-rs/tui/src/bottom_pane/mod.rs

@@ -1123,6 +1123,11 @@ impl BottomPane {
         self.request_redraw();
     }
 
+    pub(crate) fn set_context_window_pending(&mut self, pending: bool) {
+        self.composer.set_context_window_pending(pending);
+        self.request_redraw();
+    }
+
     /// Show a generic list selection view with the provided items.
     pub(crate) fn show_selection_view(
         &mut self,