perf(tui): unified Ink cache eviction on memory pressure + session reset

Adds an `evictInkCaches(level)` API that prunes the four hot module-level
caches (`widthCache`, `wrapCache`, `sliceCache`, `lineWidthCache`) with
either a half-keep LRU pass or a full clear. Wired into:

- memoryMonitor: half-prune on 'high', full drop on 'critical', before
  the heap dump / auto-restart path. Gives long sessions a shot at
  recovering RSS instead of hard-exiting.
- useSessionLifecycle.resetSession: half-prune so a /new session starts
  with a half-warm pool and the prior session can resume cheaply.

Also: lineWidthCache now uses LRU half-eviction on overflow instead of a
full `cache.clear()`, matching the other three caches.

Comparison vs claude-code: both forks now share the same `prevScreen`
blit + dirty-cascade machinery in render-node-to-output. Their smoothness
came from sibling-memo discipline (every chrome pane memo'd so dirty
cascade doesn't disable transcript blit) — already in place in our
appLayout.tsx (TranscriptPane / ComposerPane / StatusRulePane all memo'd).
Alt-screen is not the cause; both use it. The remaining gap was per-row
CPU on width/wrap/slice, which the previous commit closed.

ui-tui/packages/hermes-ink/src/entry-exports.ts

@@ -1,6 +1,12 @@
 export { default as useStderr } from './hooks/use-stderr.js'
 export { default as useStdout } from './hooks/use-stdout.js'
 export { Ansi } from './ink/Ansi.js'
+export {
+  evictInkCaches,
+  type EvictLevel,
+  type InkCacheSizes,
+  inkCacheSizes
+} from './ink/cache-eviction.js'
 export { AlternateScreen } from './ink/components/AlternateScreen.js'
 export { default as Box } from './ink/components/Box.js'
 export { default as Link } from './ink/components/Link.js'

ui-tui/packages/hermes-ink/src/ink/cache-eviction.ts

@@ -0,0 +1,46 @@
+// Unified cache eviction for the four hot Ink module-level caches:
+//   - widthCache       (stringWidth.ts)
+//   - wrapCache        (wrap-text.ts)
+//   - sliceCache       (sliceAnsi.ts)
+//   - lineWidthCache   (line-width-cache.ts)
+//
+// Used by the host (TUI) under memory pressure or on session swap to drop
+// content-keyed entries that won't recur. All caches are content-keyed
+// (not session-keyed), so cross-session sharing is normally beneficial —
+// only evict when memory tightens or when the user explicitly resets.
+
+import { evictLineWidthCache, lineWidthCacheSize } from './line-width-cache.js'
+import { evictWidthCache, widthCacheSize } from './stringWidth.js'
+import { evictWrapCache, wrapCacheSize } from './wrap-text.js'
+
+import { evictSliceCache, sliceCacheSize } from '../utils/sliceAnsi.js'
+
+export interface InkCacheSizes {
+  lineWidth: number
+  slice: number
+  width: number
+  wrap: number
+}
+
+export function inkCacheSizes(): InkCacheSizes {
+  return {
+    lineWidth: lineWidthCacheSize(),
+    slice: sliceCacheSize(),
+    width: widthCacheSize(),
+    wrap: wrapCacheSize()
+  }
+}
+
+export type EvictLevel = 'all' | 'half'
+
+export function evictInkCaches(level: EvictLevel = 'half'): InkCacheSizes {
+  const before = inkCacheSizes()
+  const keep = level === 'half' ? 0.5 : 0
+
+  evictWidthCache(keep)
+  evictWrapCache(keep)
+  evictSliceCache(keep)
+  evictLineWidthCache(keep)
+
+  return before
+}

ui-tui/packages/hermes-ink/src/ink/line-width-cache.ts

@@ -11,18 +11,35 @@ export function lineWidth(line: string): number {
   const cached = cache.get(line)
 
   if (cached !== undefined) {
+    cache.delete(line)
+    cache.set(line, cached)
     return cached
   }
 
   const width = stringWidth(line)
 
-  // Evict when cache grows too large (e.g. after many different responses).
-  // Simple full-clear is fine — the cache repopulates in one frame.
   if (cache.size >= MAX_CACHE_SIZE) {
-    cache.clear()
+    cache.delete(cache.keys().next().value!)
   }
 
   cache.set(line, width)
 
   return width
 }
+
+export function lineWidthCacheSize(): number {
+  return cache.size
+}
+
+export function evictLineWidthCache(keepRatio = 0): void {
+  if (keepRatio <= 0) {
+    cache.clear()
+    return
+  }
+
+  const target = Math.floor(cache.size * keepRatio)
+
+  while (cache.size > target) {
+    cache.delete(cache.keys().next().value!)
+  }
+}

ui-tui/packages/hermes-ink/src/ink/stringWidth.ts

@@ -325,3 +325,20 @@ export const stringWidth: (str: string) => number = str => {
 
   return w
 }
+
+export function widthCacheSize(): number {
+  return widthCache.size
+}
+
+export function evictWidthCache(keepRatio = 0): void {
+  if (keepRatio <= 0) {
+    widthCache.clear()
+    return
+  }
+
+  const target = Math.floor(widthCache.size * keepRatio)
+
+  while (widthCache.size > target) {
+    widthCache.delete(widthCache.keys().next().value!)
+  }
+}

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

@@ -107,3 +107,20 @@ export default function wrapText(text: string, maxWidth: number, wrapType: Style
 
   return memoizedWrap(text, maxWidth, wrapType)
 }
+
+export function wrapCacheSize(): number {
+  return wrapCache.size
+}
+
+export function evictWrapCache(keepRatio = 0): void {
+  if (keepRatio <= 0) {
+    wrapCache.clear()
+    return
+  }
+
+  const target = Math.floor(wrapCache.size * keepRatio)
+
+  while (wrapCache.size > target) {
+    wrapCache.delete(wrapCache.keys().next().value!)
+  }
+}

ui-tui/packages/hermes-ink/src/utils/sliceAnsi.ts

@@ -45,6 +45,23 @@ export default function sliceAnsi(str: string, start: number, end?: number): str
   return computeSlice(str, start, end)
 }
 
+export function sliceCacheSize(): number {
+  return sliceCache.size
+}
+
+export function evictSliceCache(keepRatio = 0): void {
+  if (keepRatio <= 0) {
+    sliceCache.clear()
+    return
+  }
+
+  const target = Math.floor(sliceCache.size * keepRatio)
+
+  while (sliceCache.size > target) {
+    sliceCache.delete(sliceCache.keys().next().value!)
+  }
+}
+
 function computeSlice(str: string, start: number, end?: number): string {
   const tokens = tokenize(str)
   let activeCodes: AnsiCode[] = []