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/lib/memoryMonitor.ts

@@ -1,3 +1,5 @@
+import { evictInkCaches } from '@hermes/ink'
+
 import { type HeapDumpResult, performHeapDump } from './memory.js'
 
 export type MemoryLevel = 'critical' | 'high' | 'normal'
@@ -39,6 +41,12 @@ export function startMemoryMonitor({
       return
     }
 
+    // Defensive eviction: prune Ink content caches before dumping/exiting.
+    // 'high' = half-prune (still warm enough to recover quickly);
+    // 'critical' = full drop. Reduces post-dump RSS and gives the user a
+    // chance to keep running rather than auto-restart.
+    evictInkCaches(level === 'critical' ? 'all' : 'half')
+
     dumped.add(level)
     const dump = await performHeapDump(level === 'critical' ? 'auto-critical' : 'auto-high').catch(() => null)