Claude text_delta stream events were dropped, so assistant text appeared
only when the completed snapshot arrived. Stream text blocks through the
existing text-delta coalescer under `${message.id}:text:${n}`, mirroring
thinking blocks, and let the snapshot complete that same item instead of
emitting a second uuid-keyed message. Turn finalization flushes partial
text on cancellation, and unstreamed snapshot blocks keep the old path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>