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/tests/markdown-incremental.spec.tsx

@@ -12,7 +12,15 @@ import { parseGfm } from '../src/markdown/parse.ts'
 
 afterEach(cleanup)
 
-/** A many-block document exercising every freeze-sensitive construct. */
+/**
+ * A many-block document exercising every freeze-sensitive construct. The
+ * prefix-equivalence property below holds only while no reference or
+ * footnote definition lands on the far side of a freeze boundary from its
+ * use: a fresh mount parses everything in one tree while the live stream's
+ * frozen blocks are already baked (the fingerprint test demonstrates the
+ * documented deviation). Keep definitions adjacent to their references when
+ * extending this corpus.
+ */
 const STREAM_DOC = [
   '# Title',
   '',