perf: halve stream flush cadence from 60hz to 30hz

FLUSH_FRAME_MS 16 -> 33. Delta events keep coalescing, just into larger
windows. AssistantTextPart already throttles text at 100ms, so the extra
granularity produced no visible benefit — just extra store.setState calls
and subscriber notifications per second.

packages/ui/src/sync/event-pipeline.ts

@@ -19,7 +19,7 @@ export type QueuedEvent = {
 
 export type FlushHandler = (events: QueuedEvent[]) => void
 
-const FLUSH_FRAME_MS = 16
+const FLUSH_FRAME_MS = 33
 const STREAM_YIELD_MS = 8
 const RECONNECT_DELAY_MS = 250
 const HEARTBEAT_TIMEOUT_MS = 15_000