fix: address markdown renderer review findings

Restore the replaced pipeline's synthetic fence newline so a real trailing
blank line inside a fence survives CodeBlock's display trim, and pin it
plus header-only tables in the DOM-parity corpus. Route
extractMarkdownPlainText through parseGfm so the grammar has one source.
Document the fixture provenance check (all 46 fixtures reproduce
byte-identically from react-markdown at 9e8101b800), the deliberate
O(prefix) divergence memcmp, the per-arm scope of grammar consistency,
the React 18 MathML namespace limitation, and the prefix-equivalence
corpus constraint.

packages/client/ui-primitives/src/markdown/parse.ts

@@ -1,9 +1,11 @@
 /**
- * The markdown renderer's two mdast grammars, one per rendering arm. Both are
- * built from the same micromark extensions, so block boundaries and inline
- * semantics are identical wherever a document (or a document tail) is parsed:
- * the incremental streaming path, the settled path, and the plain-text
- * projection all agree on where blocks start and end.
+ * The markdown renderer's two mdast grammars, one per rendering arm. Each
+ * arm is internally consistent — the incremental tail parses, the one-shot
+ * parses, and the plain-text projection of a given grammar always agree on
+ * where blocks start and end — and the settled grammar is the streaming one
+ * plus the math extensions, so the arms differ only where TeX delimiters
+ * begin a math construct (a `$$` block is a paragraph while streaming and a
+ * math block once settled, by design).
  */
 
 import type { Root } from 'mdast'