Ports the jitter-buffer approach from coder/mux to smooth out bursty
LLM token delivery into steady character-by-character reveal.

SmoothText.ts contains:
- SmoothTextEngine: pure state machine with adaptive-rate budget model
  (72-420 cps, frame-rate invariant, max 120 char visual lag)
- useSmoothStreamingText: React hook bridging the engine via RAF loop
  with single-integer state, grapheme-safe slicing, and self-terminating
  animation
- STREAM_SMOOTHING constants for tuning

SmoothText.test.ts covers all 8 engine behaviors including frame-rate
invariance across 60Hz/240Hz displays.

Adds SmoothedResponse component that wraps <Response> with the
useSmoothStreamingText hook during live streaming. Historical
messages bypass smoothing and render through <Response> directly.

The integration is minimal: renderBlockList's 'response' case now
branches on isStreaming to pick SmoothedResponse vs plain Response.

… SmoothText (#22509)

## Summary

Move RAF loop ownership and subscriber management into
`SmoothTextEngine`. The hook now uses `useMemo` (keyed on `streamKey`)
and `useSyncExternalStore` instead of two `useEffect`s, `useState`, and
six `useRef`s.

## What changed

### `SmoothTextEngine` gained:
- `listeners: Set<() => void>` — subscriber tracking
- `rafId` / `previousTimestamp` — moved from the hook's refs into the
engine
- `subscribe(listener)` — adds a listener, returns unsubscribe
(`useSyncExternalStore` contract)
- `dispose()` — stops loop, clears listeners
- `startLoop()` / `stopLoop()` — private RAF lifecycle
- `frame` — the RAF callback, previously a ref-stored closure in the
hook
- `notify()` — calls all listeners
- `update()` now calls `startLoop()`/`stopLoop()`/`notify()` as side
effects of state changes
- `reset()` now calls `stopLoop()`

### `useSmoothStreamingText` removed:
- `useState` (visibleLength)
- Both `useEffect`s (re-arm + lifecycle)
- 6 refs: `engineRef`, `previousStreamKeyRef`, `visibleLengthRef`,
`rafIdRef`, `previousTimestampRef`, `frameRef`
- Both `biome-ignore` suppressions

### `useSmoothStreamingText` replaced with:
- `useMemo(() => new SmoothTextEngine(), [streamKey])` — engine creation
+ reset on key change
- `useSyncExternalStore(engine.subscribe, () => engine.visibleLength)` —
reactive snapshot

Everything else (constants, `clamp`, `getAdaptiveRate`, `tick()`
internals, grapheme segmenter, `sliceAtGraphemeBoundary`, types, return
value shape) is identical.

## Testing

Existing `SmoothTextEngine` unit tests pass without modification.