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/src/lib/virtualHeights.ts

@@ -0,0 +1,58 @@
+import type { Msg } from '../types.js'
+
+import { boundedHistoryRenderText } from './text.js'
+
+export const hashText = (text: string) => {
+  let h = 5381
+
+  for (let i = 0; i < text.length; i++) {
+    h = ((h << 5) + h) ^ text.charCodeAt(i)
+  }
+
+  return (h >>> 0).toString(36)
+}
+
+export const messageHeightKey = (msg: Msg) =>
+  [msg.role, msg.kind ?? '', hashText([msg.text, msg.thinking ?? '', msg.tools?.join('\n') ?? ''].join('\0'))].join(':')
+
+export const wrappedLines = (text: string, width: number) => {
+  const w = Math.max(1, width)
+
+  return text.split('\n').reduce((n, line) => n + Math.max(1, Math.ceil(line.length / w)), 0)
+}
+
+export const estimatedMsgHeight = (
+  msg: Msg,
+  cols: number,
+  { compact, details, limitHistory = false }: { compact: boolean; details: boolean; limitHistory?: boolean }
+) => {
+  if (msg.kind === 'intro') {
+    return msg.info?.version ? 9 : 5
+  }
+
+  if (msg.kind === 'panel') {
+    return Math.max(3, (msg.panelData?.sections.length ?? 1) * 2 + 1)
+  }
+
+  const bodyWidth = Math.max(20, cols - 5)
+  const text = msg.role === 'assistant' && limitHistory ? boundedHistoryRenderText(msg.text) : msg.text
+  let h = wrappedLines(text || ' ', bodyWidth)
+
+  if (!compact && msg.role === 'assistant') {
+    h += Math.min(6, (text.match(/\n\s*\n/g) ?? []).length)
+  }
+
+  if (details) {
+    h += (msg.tools?.length ?? 0) + wrappedLines(msg.thinking ?? '', bodyWidth)
+  }
+
+  if (msg.role === 'user' || msg.kind === 'slash' || msg.kind === 'diff') {
+    h++
+  }
+
+  if (msg.role === 'user' || msg.kind === 'diff') {
+    h++
+  }
+
+  return Math.max(1, h)
+}