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'