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/packages/hermes-ink/src/ink/output.ts

@@ -467,15 +467,15 @@ export default class Output {
 
             if (clipHorizontally) {
               lines = lines.map(line => {
-                const startsBefore = x < clip.x1!
                 const width = stringWidth(line)
+                const startsBefore = x < clip.x1!
                 const endsAfter = x + width > clip.x2!
 
                 // Fast path: line fits entirely within the clip box — skip
-                // the tokenize/slice. This is the common case for transcript
-                // text where containers are wider than the rendered content.
-                // CPU profile (Apr 2026) showed sliceAnsi at 18% total time;
-                // most calls were no-op slices like (line, 0, width).
+                // tokenize/slice. Common case for transcript text where
+                // containers are wider than rendered content. CPU profile
+                // (Apr 2026): sliceAnsi at 18% total during scroll, mostly
+                // no-op (line, 0, width) slices.
                 if (!startsBefore && !endsAfter) {
                   return line
                 }