F10 SLICE-2: extract colorByAuthor from renderAuthorship sentinels (#29)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/trackChangesModel.ts

@@ -351,6 +351,22 @@ function sentinelsToSpans(html: string): string {
     .split(SENT.human.close).join("</span>");
 }
 
+/**
+ * Color one prose block's HTML by F3 author spans (PUA-sentinel technique,
+ * salvaged from F9 renderAuthorship). `blockStart` is the block's char offset in
+ * the source so spans map correctly. Pure; deterministic.
+ */
+export function colorByAuthor(
+  raw: string,
+  blockStart: number,
+  spans: AuthorSpan[],
+  render: (src: string) => string,
+): string {
+  const overlapping = spans.filter((s) => s.end > blockStart && s.start < blockStart + raw.length);
+  const injected = injectSentinels(raw, blockStart, overlapping);
+  return sentinelsToSpans(render(injected));
+}
+
 /**
  * Pure authorship render (INV-26/28): the CURRENT text with each F3-attributed
  * span colored by author. Prose blocks get inline `<span class="cw-by-*">`;
@@ -379,8 +395,7 @@ export function renderAuthorship(
         if (!badge) return `<div class="cw-blk">${inner}</div>`;
         return `<div class="cw-blk ${badge.cls}"><span class="cw-badge">${badge.label}</span>${inner}</div>`;
       }
-      const injected = injectSentinels(b.raw, b.start, overlapping);
-      return `<div class="cw-blk">${sentinelsToSpans(safe(injected))}</div>`;
+      return `<div class="cw-blk">${colorByAuthor(b.raw, b.start, overlapping, safe)}</div>`;
     })
     .join("\n");
 }