feat(tui): HERMES_TUI_FPS=1 shows live fps counter

Adds a corner-overlay FPS readout gated on HERMES_TUI_FPS, fed by
ink's onFrame callback (so it's the REAL render rate, not a timer).
Displays fps, last-frame duration, and total frame count, colored by
threshold (green ≥50, yellow ≥30, red below).

Implementation:
  * lib/fpsStore.ts — nanostore atom updated from a trackFrame()
    sink.  Ring buffer of last 30 frame timestamps; fps = 29/elapsed.
    trackFrame is undefined when SHOW_FPS is off so ink's onFrame
    short-circuits at the optional chain.
  * components/fpsOverlay.tsx — tiny <Text> subscriber; returns null
    when SHOW_FPS is off (React skips the subtree entirely).
  * entry.tsx — composes onFrame from logFrameEvent (dev-perf) and
    trackFrame (fps) so both flags can coexist.  When both are off,
    onFrame is undefined and ink never attaches the handler.
  * appLayout.tsx — mounts the overlay as a flex-shrink=0 right-
    aligned Box below the composer, conditional on SHOW_FPS.

Usage:
  HERMES_TUI_FPS=1 hermes --tui
  # bottom right: "  62.3fps ·   0.8ms · #1234" (green/yellow/red)

Intended as a user-facing diagnostic during the scroll-perf tuning
pass — watch the counter drop while holding PageUp to see where
frames go silent, without having to run scripts/profile-tui.py in a
side terminal.

126 files post-compile with React Compiler; 352 tests still pass.

ui-tui/src/components/fpsOverlay.tsx

@@ -0,0 +1,48 @@
+// FPS counter overlay — renders in the bottom-right corner when
+// HERMES_TUI_FPS=1.  Zero-cost when disabled (returns null at the
+// top of the component; React skips the whole subtree).
+//
+// Subscribes to $fpsState via nanostores.  The store is only updated
+// when the env flag is on (trackFrame is undefined otherwise), so we
+// also gate the subscription on SHOW_FPS to avoid a useless listener.
+
+import { Text } from '@hermes/ink'
+import { useStore } from '@nanostores/react'
+
+import { SHOW_FPS } from '../config/env.js'
+import { $fpsState } from '../lib/fpsStore.js'
+
+const fpsColor = (fps: number) => {
+  if (fps >= 50) {
+    return 'green'
+  }
+
+  if (fps >= 30) {
+    return 'yellow'
+  }
+
+  return 'red'
+}
+
+export function FpsOverlay() {
+  if (!SHOW_FPS) {
+    return null
+  }
+
+  return <FpsOverlayInner />
+}
+
+function FpsOverlayInner() {
+  const { fps, lastDurationMs, totalFrames } = useStore($fpsState)
+
+  // Zero-pad to stable width so the corner doesn't jitter as digits
+  // come and go.  Format: " 62fps  0.3ms #12345"
+  const fpsStr = fps.toFixed(1).padStart(5)
+  const durStr = lastDurationMs.toFixed(1).padStart(5)
+
+  return (
+    <Text color={fpsColor(fps)}>
+      {fpsStr}fps · {durStr}ms · #{totalFrames}
+    </Text>
+  )
+}