Add Fast mode status-line indicator (#13670)

Addresses feature request #13660

Adds new option to `/statusline` so the status line can display "fast
on" or "fast off"

Summary
- introduce a `FastMode` status-line item so `/statusline` can render
explicit `Fast on`/`Fast off` text for the service tier
- wire the item into the picker metadata and resolve its string from
`ChatWidget` without adding any unrelated `thread-name` logic or storage
changes
- ensure the refresh paths keep the cached footer in sync when the
service tier (fast mode) changes

Testing
- Manually tested

Here's what it looks like when enabled:

<img width="366" height="75" alt="image"
src="https://github.com/user-attachments/assets/7f992d2b-6dab-49ed-aa43-ad496f56f193"
/>

codex-rs/tui/src/chatwidget/tests.rs

@@ -1951,6 +1951,10 @@ fn lines_to_single_string(lines: &[ratatui::text::Line<'static>]) -> String {
     s
 }
 
+fn status_line_text(chat: &ChatWidget) -> Option<String> {
+    chat.bottom_pane.status_line_text()
+}
+
 fn make_token_info(total_tokens: i64, context_window: i64) -> TokenUsageInfo {
     fn usage(total_tokens: i64) -> TokenUsage {
         TokenUsage {
@@ -9282,6 +9286,39 @@ async fn status_line_branch_refreshes_after_interrupt() {
     assert!(chat.status_line_branch_pending);
 }
 
+#[tokio::test]
+async fn status_line_fast_mode_renders_on_and_off() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;
+    chat.config.tui_status_line = Some(vec!["fast-mode".to_string()]);
+
+    chat.refresh_status_line();
+    assert_eq!(status_line_text(&chat), Some("Fast off".to_string()));
+
+    chat.set_service_tier(Some(ServiceTier::Fast));
+    chat.refresh_status_line();
+    assert_eq!(status_line_text(&chat), Some("Fast on".to_string()));
+}
+
+#[tokio::test]
+async fn status_line_fast_mode_footer_snapshot() {
+    use ratatui::Terminal;
+    use ratatui::backend::TestBackend;
+
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;
+    chat.show_welcome_banner = false;
+    chat.config.tui_status_line = Some(vec!["fast-mode".to_string()]);
+    chat.set_service_tier(Some(ServiceTier::Fast));
+    chat.refresh_status_line();
+
+    let width = 80;
+    let height = chat.desired_height(width);
+    let mut terminal = Terminal::new(TestBackend::new(width, height)).expect("create terminal");
+    terminal
+        .draw(|f| chat.render(f.area(), f.buffer_mut()))
+        .expect("draw fast-mode footer");
+    assert_snapshot!("status_line_fast_mode_footer", terminal.backend());
+}
+
 #[tokio::test]
 async fn stream_recovery_restores_previous_status_header() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;