The markdown coalescing loop was processing chunks back-to-back without
yielding to the browser's paint cycle. At high token rates (250+ tok/s),
this caused complete UI freeze as the main thread was perpetually busy.
Add a requestAnimationFrame yield between processing batches. This allows
the browser to paint at screen FPS regardless of token throughput. Chunks
arriving during the yield are coalesced and processed together, so we
skip intermediate states and jump straight to the latest content.
Before: Chunk->process->Chunk->process->... (browser never paints = freeze)
After: Chunk->process->[RAF]->coalesced chunks->process->[RAF]->... (screen FPS)
Tested with 250 tok/s streams on 50K+ token contexts: smooth scrolling
and responsive UI throughout.
1c9085c9a6