perf(tui): debounce resize RPC + column-aware useVirtualHistory

VSCode panel-drag fires 20+ SIGWINCHes/sec, each previously triggering
an unthrottled `terminal.resize` gateway RPC and a full transcript
re-virtualization with stale per-row height cache.

## Changes

### gateway RPC debounce (ui-tui/src/app/useMainApp.ts)
- `terminal.resize` RPC now trailing-debounced at 100 ms. React `cols`
  state stays synchronous (needed for Yoga / in-process rendering),
  only the round-trip to Python coalesces. Prevents gateway flood
  during panel-drag / tmux-pane-resize.

### column-aware useVirtualHistory (ui-tui/src/hooks/useVirtualHistory.ts)
- New required `columns` param, plumbed through from useMainApp.
- On column change: scale every cached row height by `oldCols/newCols`
  (Math.max 1, Math.round) instead of clearing. Clearing forces a
  pessimistic back-walk that mounts ~190 rows at once (viewport + 2x
  overscan at 1-row estimate), each a fresh marked.lexer + syntax
  highlight ≈ 3 ms — ~600 ms React commit block. Scaled heights keep
  the back-walk tight.
- `freezeRenders=2`: reuse pre-resize mount range for 2 renders so
  already-mounted MessageRows keep their warm useMemo results. Without
  this the first post-resize render would unmount + remount most rows
  (pessimistic coverage) = visible flash + 150 ms+ freeze.
- `skipMeasurement` flag: first post-resize useLayoutEffect would read
  PRE-resize Yoga heights (Yoga's stored values are still from the
  frame before this render's calculateLayout with new width) and
  poison the scaled cache. Skip the measurement loop for that one
  render; next render's Yoga is correct.

## Validation
- tsc `--noEmit` clean
- eslint clean on touched files
- `vitest run`: 15 files / 102 tests passing

The renderer-level resize patterns (sync-dim-capture + microtask-
coalesced React commit, atomic BSU/ESU erase-before-paint, mouse-
tracking reassert) already live in hermes-ink's own `handleResize`;
this patch adds the matching app-layer hygiene.

ui-tui/src/app/useMainApp.ts

@@ -161,7 +161,7 @@ export function useMainApp(gw: GatewayClient) {
     [historyItems, messageId]
   )
 
-  const virtualHistory = useVirtualHistory(scrollRef, virtualRows)
+  const virtualHistory = useVirtualHistory(scrollRef, virtualRows, cols)
 
   const scrollWithSelection = useCallback(
     (delta: number) => {
@@ -306,12 +306,26 @@ export function useMainApp(gw: GatewayClient) {
       return
     }
 
-    const onResize = () =>
-      rpc<TerminalResizeResponse>('terminal.resize', { cols: stdout.columns ?? 80, session_id: ui.sid })
+    let timer: null | ReturnType<typeof setTimeout> = null
+
+    const onResize = () => {
+      if (timer) {
+        clearTimeout(timer)
+      }
+
+      timer = setTimeout(() => {
+        timer = null
+        void rpc<TerminalResizeResponse>('terminal.resize', { cols: stdout.columns ?? 80, session_id: ui.sid })
+      }, 100)
+    }
 
     stdout.on('resize', onResize)
 
     return () => {
+      if (timer) {
+        clearTimeout(timer)
+      }
+
       stdout.off('resize', onResize)
     }
   }, [rpc, stdout, ui.sid])