Re-parsing the full message markdown every reveal frame is O(N^2) over a
long answer and dominated stream CPU.
- Throttle useSmoothReveal commits to ~1 frame (REVEAL_MIN_COMMIT_MS).
- Memoize block parsing with an LRU keyed on source text so only changed
blocks re-parse.
- Replace Streamdown's full-text parseIncompleteMarkdown with a
tail-bounded remend: scan to the last top-level boundary outside
fences/math and repair only the trailing open block. New remend-tail.ts
is proven render-equivalent to full remend at every streaming prefix
(remend-tail.test.ts), minus an intentional, documented divergence on
cross-block dangling openers.
