docs: clarify wrapForFrac and streaming math-fence rationale

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

ui-tui/src/components/streamingMarkdown.tsx

@@ -42,6 +42,18 @@ import { Md } from './markdown.js'
 // snippets like ` ```\n$$x$$\n``` ` (math example inside a code block)
 // don't double-count. A `$$x$$` line that opens AND closes on its own
 // produces zero net toggles; that's `len >= 4` plus `endsDollar`.
+//
+// NB: this is INTENTIONALLY more conservative than `markdown.tsx`'s
+// parser, which falls back to paragraph rendering when an `$$` opener
+// has no matching closer. The renderer can do that safely because it
+// always sees the full text on every call. The streaming chunker
+// cannot — once a chunk is committed to the monotonic stable prefix it
+// is frozen, so prematurely deciding "this `$$` is just prose" would
+// permanently commit a paragraph rendering that becomes wrong the
+// instant the closer streams in. Treating any unmatched `$$` opener
+// as still-open keeps the boundary parked behind it until the closer
+// arrives (or the stream ends and the non-streaming `<Md>` takes over,
+// at which point the renderer's fallback kicks in correctly).
 const fenceOpenAt = (s: string, end: number) => {
   let codeOpen = false
   let mathOpen = false