perf(ui-gpui): add render counters, remove redundant cx.observe, reduce repaints

- BlinkState.start_from_context<V>() uses Context<V> spawn (not App)
  which correctly registers the async task in the window's executor
- ProjectGrid NOT cached (blocks child dirty propagation)
- AgentPane + TerminalView cached with into_cached_flex()
- Render diagnostic: ProjectBox renders 2x/sec (blink), both boxes
  re-render because shared parent, but inner cached views are free
- CPU: ~3% with pulsing dot visible

ui-gpui/src/components/blink_state.rs

@@ -17,17 +17,23 @@ impl BlinkState {
         Self { visible: true, epoch: 0 }
     }
 
-    pub fn start(entity: &Entity<Self>, cx: &mut App) {
+    pub fn start_from_context<V: 'static>(entity: &Entity<Self>, cx: &mut Context<V>) {
         let weak = entity.downgrade();
-        cx.spawn(async move |cx: &mut AsyncApp| {
+        eprintln!("[BlinkState] Starting blink timer");
+        cx.spawn(async move |_parent: WeakEntity<V>, cx: &mut AsyncApp| {
+            eprintln!("[BlinkState] Spawn executing — entering blink loop");
             loop {
                 cx.background_executor().timer(Duration::from_millis(500)).await;
                 let ok = weak.update(cx, |state, cx| {
                     state.visible = !state.visible;
                     state.epoch += 1;
+                    eprintln!("[BlinkState] Toggled visible={}", state.visible);
                     cx.notify();
                 });
-                if ok.is_err() { break; }
+                if ok.is_err() {
+                    eprintln!("[BlinkState] Entity dropped, stopping blink");
+                    break;
+                }
             }
         }).detach();
     }