fix(tui): bound retained state against idle OOM

Guards four unbounded growth paths reachable at idle — the shape matches
reports of the TUI hitting V8's 2GB heap limit after ~1m of idle with 0
tokens used (Mark-Compact freed ~6MB of 2045MB → pure retention).

- `GatewayClient.logs` + `gateway.stderr` events: 200-line cap is bytes-
  uncapped; a chatty Python child emitting multi-MB lines (traceback,
  dumped config, unsplit JSON) retains everything. Truncate at 4KB/line.
- `GatewayClient.bufferedEvents`: unbounded until `drain()` fires. Cap
  at 2000 so a pre-mount event storm can't pin memory indefinitely.
- `useMainApp` gateway `exit` handler: didn't reset `turnController`, so
  a mid-stream crash left `bufRef`/`reasoningText` alive forever.
- `pasteSnips` count-capped (32) but byte-uncapped. Add a 4MB total cap
  and clear snips in `clearIn` so submitted pastes don't linger.
- `StylePool.transitionCache`: uncapped `Map<number,string>`. Full-clear
  at 32k entries (mirrors `charCache` pattern).

ui-tui/src/app/useComposerState.ts

@@ -16,6 +16,28 @@ import { pasteTokenLabel, stripTrailingPasteNewlines } from '../lib/text.js'
 import type { PasteSnippet, UseComposerStateOptions, UseComposerStateResult } from './interfaces.js'
 import { $isBlocked } from './overlayStore.js'
 
+const PASTE_SNIP_MAX_COUNT = 32
+const PASTE_SNIP_MAX_TOTAL_BYTES = 4 * 1024 * 1024
+
+const trimSnips = (snips: PasteSnippet[]): PasteSnippet[] => {
+  let total = 0
+  const out: PasteSnippet[] = []
+
+  for (let i = snips.length - 1; i >= 0; i--) {
+    const snip = snips[i]!
+    const size = snip.text.length
+
+    if (out.length >= PASTE_SNIP_MAX_COUNT || total + size > PASTE_SNIP_MAX_TOTAL_BYTES) {
+      break
+    }
+
+    total += size
+    out.unshift(snip)
+  }
+
+  return out.length === snips.length ? snips : out
+}
+
 export function useComposerState({ gw, onClipboardPaste, submitRef }: UseComposerStateOptions): UseComposerStateResult {
   const [input, setInput] = useState('')
   const [inputBuf, setInputBuf] = useState<string[]>([])
@@ -31,6 +53,7 @@ export function useComposerState({ gw, onClipboardPaste, submitRef }: UseCompose
   const clearIn = useCallback(() => {
     setInput('')
     setInputBuf([])
+    setPasteSnips([])
     setQueueEdit(null)
     setHistoryIdx(null)
     historyDraftRef.current = ''
@@ -68,7 +91,7 @@ export function useComposerState({ gw, onClipboardPaste, submitRef }: UseCompose
       const tail = cursor < value.length && !/\s/.test(value[cursor] ?? '') ? ' ' : ''
       const insert = `${lead}${label}${tail}`
 
-      setPasteSnips(prev => [...prev, { label, text: cleanedText }].slice(-32))
+      setPasteSnips(prev => trimSnips([...prev, { label, text: cleanedText }]))
 
       void gw
         .request<{ path?: string }>('paste.collapse', { text: cleanedText })

ui-tui/src/app/useMainApp.ts

@@ -448,6 +448,7 @@ export function useMainApp(gw: GatewayClient) {
     const handler = (ev: GatewayEvent) => onEventRef.current(ev)
 
     const exitHandler = () => {
+      turnController.reset()
       patchUiState({ busy: false, sid: null, status: 'gateway exited' })
       turnController.pushActivity('gateway exited · /logs to inspect', 'error')
       sys('error: gateway exited')