Address two Copilot review comments on PR #17175.
- `wrapForFrac` doc said "additive operators or whitespace" but the
implementation also matches `*` and `/`. The wider behaviour is the
one we want (nested products and fractions need parens to disambiguate
inline `/`), so the doc is updated to match instead of tightening the
regex.
- `fenceOpenAt` was flagged as "overly conservative" vs. `markdown.tsx`,
which falls back to paragraph rendering for unclosed `$$` openers.
Mirroring that fallback in the streaming chunker would prematurely
commit a paragraph rendering of the unclosed opener to the monotonic
stable prefix, where it would be frozen and become wrong the moment
the closer streams in. The asymmetry is deliberate; document why so
it isn't "fixed" again later.
Made-with: Cursor
