Contribute rewrite Phase 3: tracked-change overlay in MarkdownPreview

Reintroduces the §8.10 inline tracked-change layer Phase 1 dropped when
Tiptap left, this time anchored to the rendered preview rather than a
writable editor. Each accepted change on the branch decorates the
preview DOM with a `<span class="tracked-insert">` at the proposed text
and a `<span class="tracked-delete">` strikethrough for the original;
hover surfaces the same ChangeTooltip DiffView uses (badge + prompt +
quote + reason), now extracted to its own file so both surfaces share
the affordance.

Design calls per the Phase 3 prompt's open list:
  • Contribute pane: default-on. The pane exists for editorial review
    of your own work; the overlay reinforces that.
  • Discuss pane: opt-in via a new "Show tracked changes" toolbar
    toggle on non-main branches. Clean prose stays the default.
  • DiffView's "Review changes" toolbar is untouched — DiffView dies in
    Phase 7 and overloading its toggle now creates surface to unwind.

Pre-fancy stance on the known overlay subtleties:
  • Drift — if `proposed` no longer appears verbatim (later manual
    edit touched it) we skip the decoration rather than mis-anchor.
  • Overlap — decorate in `acted_at` ascending order; later spans win
    on whatever text is still un-wrapped.
  • Mermaid — text nodes inside `.mermaid-block` are skipped; a tracked
    span that intersects diagram source drops silently. Flagged as a
    known limitation rather than worked around.
  • Code — `<pre>`/`<code>` parents skipped too; matching inside
    verbatim code produced noisy false positives in fixtures.

Backend tests: 125 passed. Helper verified via Vite preview sandbox
eval across eight cases (single-paragraph match, distinct-original
strike, no-match skip, mermaid skip, two-change overlap ordering,
declined/pending ignored, whitespace-normalized cross-newline match,
code-block skip), plus computed-style proof for the new
`.markdown-preview .tracked-insert` / `.tracked-delete` rules. The
end-to-end CM6 → accept → overlay flow against a live branch wasn't
exercised (backend not running this session); the simpler unit-level
verification looked clean, but a future session with the backend up
should drive that golden path before Phase 4 piles on top.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/RFCView.jsx

@@ -94,6 +94,11 @@ export default function RFCView({ viewer }) {
   const [selection, setSelection] = useState(null)
   const [reviewMode, setReviewMode] = useState(false)
   const [reviewHTML, setReviewHTML] = useState('')
+  // Phase 3 — Discuss-mode opt-in for the §8.10 tracked-change overlay
+  // in the single-pane preview. Contribute mode is default-on (the
+  // pane exists for editorial review of your own work); Discuss mode
+  // keeps clean prose by default and exposes a toggle.
+  const [discussShowChanges, setDiscussShowChanges] = useState(false)
 
   // Mode: discuss vs contribute (§8.3). Always discuss on main.
   const [mode, setMode] = useState('discuss')
@@ -142,6 +147,7 @@ export default function RFCView({ viewer }) {
     setPendingDiscussChanges([])
     setManualPending(null)
     setReviewMode(false)
+    setDiscussShowChanges(false)
     setSelection(null)
     setMode('discuss')
 
@@ -649,6 +655,22 @@ export default function RFCView({ viewer }) {
               </span>
             </div>
           )}
+          {inDiscuss && branchParam !== 'main'
+            && changes.some(c => c.state === 'accepted') && (
+            <div className="editor-toolbar">
+              <button
+                type="button"
+                className={`btn-review-toggle${discussShowChanges ? ' active' : ''}`}
+                onClick={() => setDiscussShowChanges(v => !v)}
+                title="§8.10 — surface accepted tracked changes inline in the preview"
+              >
+                {discussShowChanges ? 'Hide tracked changes' : 'Show tracked changes'}
+              </button>
+              <span className="editor-toolbar-hint">
+                {changes.filter(c => c.state === 'accepted').length} accepted on this branch
+              </span>
+            </div>
+          )}
           {reviewMode ? (
             <DiffView html={reviewHTML} changes={changes} messages={messages} />
           ) : (
@@ -666,6 +688,9 @@ export default function RFCView({ viewer }) {
                     <MarkdownPreview
                       content={previewContent}
                       onSelectionChange={handleSelectionChange}
+                      changes={changes}
+                      messages={messages}
+                      showTrackedChanges
                     />
                   </div>
                 </div>
@@ -674,6 +699,9 @@ export default function RFCView({ viewer }) {
                   content={editorContent}
                   onSelectionChange={handleSelectionChange}
                   className="markdown-preview-solo"
+                  changes={changes}
+                  messages={messages}
+                  showTrackedChanges={inDiscuss && discussShowChanges}
                 />
               )}
               <SelectionTooltip