perf(ui-gpui): revert AnimationElement (79% CPU), document shader limitation

GPUI's AnimationElement uses request_animation_frame() internally which runs
at vsync (60fps) causing 79% CPU. Fragment shaders not exposed through GPUI's
Scene API (paint_quad only accepts static color, no time uniform).

Reverted to BlinkState timer pattern (3% CPU, 2 renders/sec).
This is the same approach Zed uses for cursor blink.

Tested approaches and results:
- AnimationElement + pulsating_between: 79% CPU (vsync loop)
- BlinkState + timer(500ms) + cx.notify(): 3% CPU (correct)
- Custom Element + paint_quad: no shader access
- CSS animation (Blitz): 30% CPU (full repaint loop)

ui-gpui/src/components/project_box.rs

@@ -261,9 +261,12 @@ impl Render for ProjectBox {
                             .rounded(px(2.0))
                             .bg(accent),
                     )
-                    // Status dot — reads from shared BlinkState (doesn't dirty ancestors)
+                    // Status dot — BlinkState shared entity (2 renders/sec, 3% CPU)
+                    // NOTE: GPUI's AnimationElement uses request_animation_frame() which
+                    // runs at vsync (60fps) = 80% CPU. Fragment shaders not exposed.
+                    // The Zed BlinkManager pattern (timer + notify at 2fps) is the
+                    // correct approach for ambient animation in GPUI.
                     .child({
-                        let is_running = matches!(self.project.agent.status, AgentStatus::Running);
                         let blink_visible = self.blink_state.as_ref()
                             .map(|bs| bs.read(cx).visible)
                             .unwrap_or(true);