createJsonRenderTransform: batch prose deltas instead of one text-delta per character

Version: @json-render/core 0.21.0

createJsonRenderTransform walks each text-delta one character at a time. Outside the line buffer, it enqueues one text-delta per character, so a 2,000-character reply becomes about 2,000 chunks on the wire and about 2,000 UI updates on the client.

Repro

Pipe text-start 1, text-delta 1 "Hello world.\n", text-end 1:

• Expected: one text-delta carrying Hello world.\n.
• Actual: thirteen text-delta chunks, one per character.

Ask

Emit the prose of a delta as one text-delta, and hold back only the part that could still turn out to be a spec line (a line that starts with {, a fence prefix, or the line in progress inside a fence).