feat: re-emit changed mermaid diagrams through mermaidDiff in the built-in preview (plan T7 §2.6, INV-29 parity)

Task 7 scoped out reusing the F7.1 (#22) intra-diagram mermaid diff in the
built-in Markdown preview's mermaid fence rendering; Task 8 then deleted the
webview that still had it, leaving mermaidDiff.ts/mermaidFlowchartDiff.ts/
mermaidSequenceDiff.ts production-dead. Wires diffMermaid into
cowritingMarkdownItPlugin's options.highlight override via a per-render
mermaid-fence queue (built in the core rule where state.env is visible,
consumed by highlight in fence-encounter order — highlight itself has no env
param) so a mermaid fence changed since the F6/F7 baseline is re-emitted with
classDef/class/linkStyle styling directives + legend before mermaid.js ever
sees it, reusing diffBlocks' baseline/current block pairing (same pairing
renderReview used). Unchanged diagrams, disabled annotations, no baseline, and
pinned-zero-diff all pass through verbatim.

Ports the substance of the 6 mermaid-diff assertions Task 8 deleted with the
webview's E2E suite as pure unit tests in previewAnnotations.test.ts instead
(DOM-free, source-level).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

media/preview-mermaid.ts

@@ -16,16 +16,20 @@
  * in-place content updates that don't reload the script (mirrors the proven
  * `bierner.markdown-mermaid` extension's own approach).
  *
- * SCOPE NOTE (Q4 finding, recorded per the migration plan's Task 7 Step 2.6):
- * this renders the CURRENT diagram only — it does NOT re-emit a changed
- * diagram through `mermaidDiff.ts`'s intra-diagram diff/legend augmentation
- * (the F7.1 feature the sealed webview preview has). That augmentation needs a
- * block-level (not word-hunk-level) diff pass wired into `annotateSource`,
- * which was judged out of scope for this increment — it cannot be verified by
- * the automated E2E harness (the built-in preview's DOM/CSP sandbox isn't
- * queryable from a host test) and shipping it unverified risked a silent
- * regression. Basic mermaid rendering (this file) IS wired and is exercised by
- * the same proven pattern as the real `bierner.markdown-mermaid` extension.
+ * RESOLVED SCOPE NOTE (Q4 finding, migration plan Task 7 Step 2.6; wired in a
+ * cross-task review follow-up): this script itself only turns `pre.mermaid`
+ * nodes into diagrams — it never sees a diff. The intra-diagram diff/legend
+ * augmentation (F7.1, INV-29..31) happens one step earlier, source-side,
+ * where it CAN be unit-tested without a DOM: `previewAnnotations.ts`'s
+ * `cowritingMarkdownItPlugin` re-emits a CHANGED mermaid fence's source
+ * through `mermaidDiff.ts` (`buildMermaidQueue`, block-level `diffBlocks`
+ * pairing against the F6/F7 baseline) before this script ever runs, so the
+ * `pre.mermaid` node this script hands to `mermaid.run` already carries the
+ * `classDef`/`class`/`linkStyle` styling directives — this script needn't (and
+ * can't, DOM-less-ly) know a diagram changed at all. Basic mermaid rendering
+ * (this file) is exercised by the same proven pattern as the real
+ * `bierner.markdown-mermaid` extension; the augmentation upstream is exercised
+ * by pure unit tests (`test/previewAnnotations.test.ts`).
  */
 import mermaid from "mermaid";