feat: authorship + change annotations in the built-in Markdown preview (D3/D21, PUC-3); cowriting.annotations toggle

Task 7 of the native-surfaces migration plan. Pure previewAnnotations.ts
(annotateSource + cowritingMarkdownItPlugin) reuses trackChangesModel.ts's
sentinel discipline (#33/#47), generalized to a 3-way ins-claude/ins-human/del
tag (exported injectSentinels/sentinelsToSpans, colorByAuthor unaffected,
84/84 existing tests pass unmodified). Host hook in extension.ts wires
registry/diffView/attribution/proposals/config into the plugin, exposed via
CowritingApi.extendMarkdownIt + a previewAnnotationHost test seam.
cowriting.annotations setting + toggleAnnotations command/menu. Q4 mermaid
(previewScripts + options.highlight fence override) implemented per the
proven bierner.markdown-mermaid pattern; intra-diagram diff augmentation
scoped out (unverifiable via the structural E2E harness) — see
task-7-report.md.

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

media/preview-annotations.css

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+/*
+ * Task 7 (D3/D21, PUC-3) — authorship + change annotations for the BUILT-IN VS
+ * Code Markdown preview (`markdown.previewStyles`). Same F10 vocabulary as the
+ * (sunsetting) custom webview's `media/preview.css`: style = operation
+ * (underline = inserted, strikethrough = removed), color = author (human
+ * green, Claude blue) — `cw-del` has no author variant (a single fixed
+ * struck-red mark; see `previewAnnotations.ts`/`trackChangesModel.ts`'s "del"
+ * sentinel tag). Light-theme defaults below; `body.vscode-dark` (also applied
+ * on high-contrast dark) overrides with the same palette the webview preview
+ * already ships, for continuity across both review surfaces.
+ */
+.cw-ins-claude {
+  background: rgba(9, 105, 218, 0.12);
+  border-bottom: 2px solid #0969da;
+  text-decoration: none;
+}
+.cw-ins-human {
+  background: rgba(26, 127, 55, 0.12);
+  border-bottom: 2px solid #1a7f37;
+  text-decoration: none;
+}
+.cw-del {
+  background: rgba(207, 34, 46, 0.1);
+  text-decoration: line-through;
+  text-decoration-color: #cf222e;
+  opacity: 0.75;
+}
+
+body.vscode-dark .cw-ins-claude,
+body.vscode-high-contrast:not(.vscode-high-contrast-light) .cw-ins-claude {
+  background: rgba(88, 166, 255, 0.15);
+  border-bottom-color: #58a6ff;
+}
+body.vscode-dark .cw-ins-human,
+body.vscode-high-contrast:not(.vscode-high-contrast-light) .cw-ins-human {
+  background: rgba(63, 185, 80, 0.14);
+  border-bottom-color: #3fb950;
+}
+body.vscode-dark .cw-del,
+body.vscode-high-contrast:not(.vscode-high-contrast-light) .cw-del {
+  background: rgba(248, 81, 73, 0.11);
+  text-decoration-color: #f85149;
+}

media/preview-mermaid.ts

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+/**
+ * Task 7 (Q4 mermaid check, D3/D21): the built-in VS Code Markdown preview does
+ * not render mermaid fences on its own — `previewAnnotations.ts`'s
+ * `cowritingMarkdownItPlugin` teaches ```mermaid fences to render as
+ * `<pre class="mermaid">SRC</pre>` (via `options.highlight`, the same
+ * extension point the built-in preview's own fence rule already calls); this
+ * script (contributed via `markdown.previewScripts`) is what actually turns
+ * those into diagrams — mermaid needs a DOM, so it runs here, in the preview's
+ * webview, never in the extension host. Bundled by esbuild as a standalone
+ * IIFE → out/media/preview-mermaid.js, so mermaid never enters the host
+ * bundle (matching the sealed webview's `media/preview.ts` precedent).
+ *
+ * Contributed preview scripts are reloaded on every content change (VS Code
+ * docs), so running at module top-level is sufficient; the
+ * `vscode.markdown.updateContent` listener is added defensively to also cover
+ * 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.
+ */
+import mermaid from "mermaid";
+
+function theme(): "dark" | "default" {
+  return document.body.classList.contains("vscode-dark") || document.body.classList.contains("vscode-high-contrast")
+    ? "dark"
+    : "default";
+}
+
+async function run(): Promise<void> {
+  const nodes = Array.from(document.querySelectorAll<HTMLElement>("pre.mermaid"));
+  if (nodes.length === 0) return;
+  mermaid.initialize({ startOnLoad: false, theme: theme(), securityLevel: "strict" });
+  try {
+    await mermaid.run({ nodes });
+  } catch {
+    // mermaid.run already marks failed nodes; ensure a visible chip per failure.
+    for (const n of nodes) {
+      if (!n.querySelector("svg")) n.setAttribute("data-cw-error", "true");
+    }
+  }
+}
+
+window.addEventListener("vscode.markdown.updateContent", () => void run());
+void run();