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/project_box.rs

@@ -110,7 +110,7 @@ impl ProjectBox {
         let should_pulse = matches!(self.project.agent.status, AgentStatus::Running);
         if should_pulse {
             let blink = cx.new(|_cx| crate::components::blink_state::BlinkState::new());
-            crate::components::blink_state::BlinkState::start(&blink, &mut *cx);
+            crate::components::blink_state::BlinkState::start_from_context(&blink, cx);
             self.blink_state = Some(blink);
         }
 
@@ -132,6 +132,9 @@ impl ProjectBox {
 
 impl Render for ProjectBox {
     fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        eprintln!("[ProjectBox::render] {} blink_visible={:?}",
+            self.project.name,
+            self.blink_state.as_ref().map(|bs| bs.read(cx).visible));
         let accent = accent_color(self.project.accent_index);
         let active_tab = self.project.active_tab;