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/workspace.rs

@@ -96,7 +96,11 @@ impl Render for Workspace {
         }
 
         // Project grid (fills remaining space) — cached with flex-1
-        main_row = main_row.child(self.project_grid.clone().into_cached_flex());
+        // ProjectGrid NOT cached — child ProjectBoxes need re-render when their
+        // BlinkState changes. Caching the grid blocks child dirty propagation.
+        main_row = main_row.child(
+            div().flex_1().h_full().child(self.project_grid.clone()),
+        );
 
         root = root.child(main_row);