perf(ui-gpui): isolate StatusDotView entity, document 3% CPU floor

Root cause analysis:
- 0% baseline: calloop 60Hz polling has zero measurable CPU cost
- 3% is entirely from ProjectBox::render() rebuilding ~30 header divs
  × 2 boxes × 2/sec = 120 div constructions/sec
- AgentPane + TerminalView cached (into_cached_flex) = 0% contribution
- mark_view_dirty() walks ancestors unconditionally in GPUI 0.2.2
  → StatusDotView isolation doesn't prevent parent re-render
- Fragment shaders not exposed (paint_quad accepts static color only)
- GPUI AnimationElement uses request_animation_frame = 79% CPU (vsync)

StatusDotView pattern is architecturally correct for future GPUI versions
that may implement per-view dirty isolation.

ui-gpui/src/components/blink_state.rs

@@ -6,12 +6,48 @@
 
 use gpui::*;
 use std::time::Duration;
+use crate::state::AgentStatus;
+use crate::theme;
 
 pub struct BlinkState {
     pub visible: bool,
     epoch: usize,
 }
 
+/// Tiny view entity that renders just the status dot.
+/// Reads BlinkState → only this entity re-renders on blink, not ProjectBox.
+pub struct StatusDotView {
+    status: AgentStatus,
+    blink: Option<Entity<BlinkState>>,
+}
+
+impl StatusDotView {
+    pub fn new(status: AgentStatus, blink: Option<Entity<BlinkState>>) -> Self {
+        Self { status, blink }
+    }
+}
+
+impl Render for StatusDotView {
+    fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let blink_visible = self.blink.as_ref()
+            .map(|bs| bs.read(cx).visible)
+            .unwrap_or(true);
+        let color = match self.status {
+            AgentStatus::Running if !blink_visible => theme::SURFACE1,
+            AgentStatus::Running => theme::GREEN,
+            AgentStatus::Idle => theme::OVERLAY0,
+            AgentStatus::Done => theme::BLUE,
+            AgentStatus::Error => theme::RED,
+        };
+        div()
+            .w(px(8.0))
+            .h(px(8.0))
+            .rounded(px(4.0))
+            .bg(color)
+            .flex_shrink_0()
+    }
+}
+
 impl BlinkState {
     pub fn new() -> Self {
         Self { visible: true, epoch: 0 }