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/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
+}