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/src/app/useSessionLifecycle.ts

@@ -1,6 +1,6 @@
 import { writeFileSync } from 'node:fs'
 
-import type { ScrollBoxHandle } from '@hermes/ink'
+import { evictInkCaches, type ScrollBoxHandle } from '@hermes/ink'
 import { type RefObject, useCallback } from 'react'
 
 import { buildSetupRequiredSections, SETUP_REQUIRED_TITLE } from '../content/setup.js'
@@ -98,6 +98,9 @@ export function useSessionLifecycle(opts: UseSessionLifecycleOptions) {
     setLastUserMsg('')
     setStickyPrompt('')
     composerActions.setPasteSnips([])
+    // Half-prune Ink content caches: new session has new keys, but a partial
+    // warm pool helps if the user resumes back to the prior session.
+    evictInkCaches('half')
   }, [composerActions, setHistoryItems, setLastUserMsg, setStickyPrompt, setVoiceProcessing, setVoiceRecording])
 
   const resetVisibleHistory = useCallback(