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>

package.json

@@ -18,6 +18,13 @@
     "onStartupFinished"
   ],
   "contributes": {
+    "markdown.markdownItPlugins": true,
+    "markdown.previewStyles": [
+      "./media/preview-annotations.css"
+    ],
+    "markdown.previewScripts": [
+      "./out/media/preview-mermaid.js"
+    ],
     "configuration": {
       "title": "Cowriting",
       "properties": {
@@ -25,6 +32,11 @@
           "type": "boolean",
           "default": true,
           "description": "When Claude is editing, reveal the \"Cowriting: Claude\" output channel (without stealing focus) as soon as Claude starts producing text, so you can read the output as it streams."
+        },
+        "cowriting.annotations": {
+          "type": "boolean",
+          "default": true,
+          "description": "Show authorship + change annotations in the Markdown preview for coedited documents."
         }
       }
     },
@@ -147,6 +159,12 @@
         "title": "✦ Make this edit",
         "category": "Cowriting",
         "icon": "$(sparkle)"
+      },
+      {
+        "command": "cowriting.toggleAnnotations",
+        "title": "Toggle Annotations",
+        "category": "Cowriting",
+        "icon": "$(eye)"
       }
     ],
     "menus": {
@@ -187,6 +205,10 @@
           "command": "cowriting.askClaude",
           "when": "editorLangId == markdown && cowriting.isCoediting"
         },
+        {
+          "command": "cowriting.toggleAnnotations",
+          "when": "editorLangId == markdown && cowriting.isCoediting"
+        },
         {
           "command": "cowriting.makeThreadEdit",
           "when": "false"
@@ -219,6 +241,11 @@
           "when": "resourceLangId == markdown && cowriting.isCoediting",
           "group": "navigation@2"
         },
+        {
+          "command": "cowriting.toggleAnnotations",
+          "when": "resourceLangId == markdown && cowriting.isCoediting",
+          "group": "navigation@3"
+        },
         {
           "command": "cowriting.markReviewed",
           "when": "resourceLangId == markdown && cowriting.isCoediting && cowriting.baselineMode == snapshot",