perf(tui): cache stringWidth/wrapText/sliceAnsi + skip-slice when line fits clip

CPU profile (Apr 2026, real-user scroll on 11k-line session) showed three
hot loops in the per-frame render path:

  Output.get() per-frame walk:                 24% total
  └─ sliceAnsi(line, from, to) per write:     18% total
  stringWidth(line) chain (cached + JS):      14% total

All three were re-doing identical work every frame: same string → same
clipped slice → same width.

Fixes:

1. Memoize stringWidth (8k-entry LRU) for non-ASCII strings; ASCII fast-path
   skips the cache (inline scan beats Map.get for short ASCII, the >90%
   case). String.charCodeAt scan up to 64 chars is cheaper than the regex
   fallback.

2. Memoize wrapText (4k-entry LRU keyed by maxWidth|wrapType|text) — wrapAnsi
   is pure and the same content reflows identically every frame.

3. Memoize sliceAnsi (4k-entry LRU keyed by start|end|str) for the
   end-defined hot path used by Output.get().

4. Skip the slice entirely in Output.get() when the line already fits the
   clip box (startsBefore=false && endsAfter=false). Most transcript lines
   never exceed their container width, and tokenizing them just to slice
   (line, 0, width) was pure overhead. This single fast-path drops
   sliceAnsi from 18% → ~0% in the profile.

Also tighten virtualization constants (MAX_MOUNTED 260→120, OVERSCAN 40→20,
SLIDE_STEP 25→12) and cap historical-message render at 800 chars / 16
lines via HISTORY_RENDER_MAX_*; messages inside the FULL_RENDER_TAIL_ITEMS
window still render in full so reading-zone behavior is unchanged.

Validation, real-user CPU profile, page-up scroll on 11k-line session:

  Output.get() self-time:     24%   →   0.3%
  sliceAnsi total:            18%   →   not in top 25
  stringWidth family:         14%   →   ~3%
  idle:                     60.7%   →  77.3%

Frame timings (synthetic page-up profile harness):
  dur p95:   ~10ms   →  4.87ms
  dur p99:   25ms+   → 12.80ms
  yoga p99:  ~20ms   →  1.87ms

The remaining CPU in the profile is Yoga layoutNode + React commit,
which is the irreducible work for this UI tree size.

ui-tui/packages/hermes-ink/src/ink/wrap-text.ts

@@ -6,6 +6,40 @@ import { wrapAnsi } from './wrapAnsi.js'
 
 const ELLIPSIS = '…'
 
+// CPU profile (Apr 2026) showed `wrap-ansi` → `string-width` consuming 30% of
+// total runtime during fast scroll: every layout pass re-wraps every visible
+// line via wrap-ansi, which calls string-width once per grapheme. The output
+// is pure of (text, maxWidth, wrapType), so memoize it. LRU-bounded so long
+// sessions don't accrete unbounded cache.
+const WRAP_CACHE_LIMIT = 4096
+const wrapCache = new Map<string, string>()
+
+function memoizedWrap(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+  // Key folds maxWidth + wrapType into the prefix so the same text re-wrapped
+  // at a different width doesn't collide. Width prefix bounded by viewport
+  // (~10 distinct widths in a session); wrapType bounded by enum (~6 values).
+  const key = `${maxWidth}|${wrapType}|${text}`
+  const cached = wrapCache.get(key)
+
+  if (cached !== undefined) {
+    // LRU touch
+    wrapCache.delete(key)
+    wrapCache.set(key, cached)
+
+    return cached
+  }
+
+  const result = computeWrap(text, maxWidth, wrapType)
+
+  if (wrapCache.size >= WRAP_CACHE_LIMIT) {
+    wrapCache.delete(wrapCache.keys().next().value!)
+  }
+
+  wrapCache.set(key, result)
+
+  return result
+}
+
 // sliceAnsi may include a boundary-spanning wide char (e.g. CJK at position
 // end-1 with width 2 overshoots by 1). Retry with a tighter bound once.
 function sliceFit(text: string, start: number, end: number): string {
@@ -42,12 +76,9 @@ function truncate(text: string, columns: number, position: 'start' | 'middle' |
   return sliceFit(text, 0, columns - 1) + ELLIPSIS
 }
 
-export default function wrapText(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+function computeWrap(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
   if (wrapType === 'wrap') {
-    return wrapAnsi(text, maxWidth, {
-      trim: false,
-      hard: true
-    })
+    return wrapAnsi(text, maxWidth, { trim: false, hard: true })
   }
 
   if (wrapType === 'wrap-char') {
@@ -55,25 +86,24 @@ export default function wrapText(text: string, maxWidth: number, wrapType: Style
   }
 
   if (wrapType === 'wrap-trim') {
-    return wrapAnsi(text, maxWidth, {
-      trim: true,
-      hard: true
-    })
+    return wrapAnsi(text, maxWidth, { trim: true, hard: true })
   }
 
   if (wrapType!.startsWith('truncate')) {
-    let position: 'end' | 'middle' | 'start' = 'end'
-
-    if (wrapType === 'truncate-middle') {
-      position = 'middle'
-    }
-
-    if (wrapType === 'truncate-start') {
-      position = 'start'
-    }
+    const position: 'end' | 'middle' | 'start' =
+      wrapType === 'truncate-middle' ? 'middle' : wrapType === 'truncate-start' ? 'start' : 'end'
 
     return truncate(text, maxWidth, position)
   }
 
   return text
 }
+
+export default function wrapText(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+  // Skip cache for trivial inputs (faster than Map lookup).
+  if (!text || maxWidth <= 0) {
+    return computeWrap(text, maxWidth, wrapType)
+  }
+
+  return memoizedWrap(text, maxWidth, wrapType)
+}