* perf(streaming): buffer SSE parts via rAF to coalesce re-renders
addStreamingPart previously called _addStreamingPartImmediate directly
for every SSE token (~50-100/sec during streaming), causing one Zustand
set() and potential re-render per token.
Now incoming parts are buffered in a module-level queue and flushed once
per animation frame. React 18's automatic batching coalesces the
synchronous set() calls within the rAF callback into a single paint.
Also handles edge cases:
- completeStreamingMessage flushes pending queue before completing
- abortCurrentOperation discards queued parts for the aborted session
- cancelAnimationFrame prevents stale flushes on abort/complete
* perf(streaming): add timeout-backed flush for rAF queue batching
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Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>
