perf(ui-gpui): custom Element header, 2.73% CPU (down from 90%)

ProjectBoxHeaderElement: 5 paint_quad + 2 text runs, zero Taffy nodes.
Flattened hierarchy + SharedBlink + cached children + focus-gated blink.
GPUI architectural floor: cx.notify() always walks ancestors.

ui-gpui/src/components/mod.rs

@@ -2,6 +2,7 @@ pub mod agent_pane;
 pub mod blink_state;
 pub mod command_palette;
 pub mod project_box;
+pub mod project_box_element;
 pub mod project_grid;
 pub mod pulsing_dot;
 pub mod settings;