chore(tui): /clean recent perf work — KISS/DRY pass

24 files, -319 LoC. Behaviour preserved, 369/369 tests green.

- hermes-ink caches: shared lruEvict helper for the four parallel LRU
  caches (stringWidth, wrapText, sliceAnsi, lineWidth); touch-on-read
  stays inlined per cache; tightened output.ts skip-slice fast path.
- wheelAccel: trimmed provenance header, collapsed env parsing, ternary
  dispatch in computeWheelStep.
- perfPane: folded ensureLogDir into once-flag, spread-with-overrides
  for fastPath/phases instead of full rebuilds.
- env: extracted truthy() (used 4×).
- virtualHeights: collapsed user/diff/slash height bumps; trail+todos
  estimate.
- useInputHandlers: scrollIdleTimer cleanup on unmount, ?? undefined
  shorthand.
- useMainApp: dropped dead liveTailVisible IIFE and liveProgress
  indirection.
- appLayout, markdown, messageLine, entry: vertical rhythm, dropped
  narration comments, inlined one-shot vars.
- fix: empty catch blocks → /* best-effort */ for no-empty lint.

ui-tui/src/components/fpsOverlay.tsx

@@ -1,10 +1,4 @@
-// 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.
+// FPS counter overlay (HERMES_TUI_FPS=1). Zero-cost when disabled.
 
 import { Text } from '@hermes/ink'
 import { useStore } from '@nanostores/react'
@@ -12,17 +6,7 @@ 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'
-}
+const fpsColor = (fps: number) => (fps >= 50 ? 'green' : fps >= 30 ? 'yellow' : 'red')
 
 export function FpsOverlay() {
   if (!SHOW_FPS) {
@@ -35,14 +19,10 @@ export function FpsOverlay() {
 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)
-
+  // Zero-pad widths so digit churn doesn't jitter the corner.
   return (
     <Text color={fpsColor(fps)}>
-      {fpsStr}fps · {durStr}ms · #{totalFrames}
+      {fps.toFixed(1).padStart(5)}fps · {lastDurationMs.toFixed(1).padStart(5)}ms · #{totalFrames}
     </Text>
   )
 }