test(tui): add stalled-pane + tiny-terminal render tests

Two more TestBackend render tests on top of the connecting/populated/empty set:
- render_stalled_pane_shows_warning_icon — covers the distinct stalled branch
  (state_icon → ⚠), which the healthy-Working test (●) didn't exercise.
- render_does_not_panic_on_tiny_terminal — renders at 20x5 to guard against
  cramped-layout panics (a classic ratatui footgun).

10 tests pass; fmt clean.

crates/colibri-glasspane-tui/src/main.rs

@@ -810,4 +810,53 @@ mod tests {
         // Must not have panicked; content is fine either way
         let _ = text;
     }
+
+    #[test]
+    fn render_stalled_pane_shows_warning_icon() {
+        // Stalled is a distinct render branch (state_icon → "⚠", magenta).
+        let snap = GlasspaneSnapshot::new(
+            "osa",
+            "2026-06-24T12:00:00Z",
+            vec![colibri_glasspane::Pane {
+                id: "pane-stalled".into(),
+                agent: "zot".into(),
+                state: AgentState::Working,
+                session_id: Some("s1".into()),
+                last_event_at: Some("2026-06-24T12:00:01Z".into()),
+                cwd: Some("/home/clawdie".into()),
+                stalled: true,
+            }],
+        );
+        let mut app = App::new(PathBuf::from("/tmp/nonexistent.sock"));
+        app.snapshot = Some(snap);
+        app.rebuild_session_list();
+        let text = render_text(&mut app, 80, 24);
+
+        assert!(text.contains("pane-stalled"), "expected pane id in: {text}");
+        // Stalled icon is ⚠ (not the ● of a healthy Working pane).
+        assert!(text.contains("⚠"), "expected stalled icon in: {text}");
+    }
+
+    #[test]
+    fn render_does_not_panic_on_tiny_terminal() {
+        // Cramped layouts are a classic ratatui panic source — the draw must
+        // still succeed (render_text's .expect would fail if it panicked).
+        let snap = GlasspaneSnapshot::new(
+            "osa",
+            "2026-06-24T12:00:00Z",
+            vec![colibri_glasspane::Pane {
+                id: "p".into(),
+                agent: "zot".into(),
+                state: AgentState::Working,
+                session_id: Some("s1".into()),
+                last_event_at: None,
+                cwd: None,
+                stalled: false,
+            }],
+        );
+        let mut app = App::new(PathBuf::from("/tmp/nonexistent.sock"));
+        app.snapshot = Some(snap);
+        app.rebuild_session_list();
+        let _ = render_text(&mut app, 20, 5);
+    }
 }