Feat/performance probe fps overlay (#472)

* feat(perf-probe): add FPS overlay toggle and tidy probe modal

Add optional rAF-based FPS readout controlled by a persisted probe flag.
Remove the separate keyboard shortcut. Collapse all phase sections by default.

* perf(fps-overlay): subscribe only to showFpsOverlay flag

Add usePerfProbeFlag so the overlay does not re-render when other probe
toggles change. Track the animation frame id with a loop-local variable.

src/components/FpsOverlay.tsx

@@ -0,0 +1,48 @@
+import { useEffect, useState } from 'react';
+import { createPortal } from 'react-dom';
+import { usePerfProbeFlag } from '../utils/perfFlags';
+
+const SAMPLE_MS = 500;
+
+/** FPS from rAF callbacks over sliding ~500ms windows; only runs when Performance Probe enables the overlay. */
+export default function FpsOverlay() {
+  const showFpsOverlay = usePerfProbeFlag('showFpsOverlay');
+  const [fps, setFps] = useState(0);
+
+  useEffect(() => {
+    if (!showFpsOverlay) {
+      setFps(0);
+      return;
+    }
+
+    let frames = 0;
+    let lastReport = performance.now();
+    let rafId = 0;
+
+    const loop = () => {
+      frames++;
+      const now = performance.now();
+      if (now - lastReport >= SAMPLE_MS) {
+        const elapsedSec = (now - lastReport) / 1000;
+        setFps(Math.round(frames / elapsedSec));
+        frames = 0;
+        lastReport = now;
+      }
+      rafId = requestAnimationFrame(loop);
+    };
+
+    rafId = requestAnimationFrame(loop);
+    return () => cancelAnimationFrame(rafId);
+  }, [showFpsOverlay]);
+
+  if (!showFpsOverlay) return null;
+
+  return createPortal(
+    <div className="fps-overlay" aria-hidden="true">
+      {fps}
+      {' '}
+      <span className="fps-overlay__unit">FPS</span>
+    </div>,
+    document.body,
+  );
+}