feat(f7): per-block error-chip fallback (PUC-6, #21)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/trackChangesModel.test.ts

@@ -122,3 +122,18 @@ describe("renderTrackChanges", () => {
     expect(a).toBe(b);
   });
 });
+
+describe("error chip (PUC-6)", () => {
+  it("a block whose render throws becomes an error chip; the rest still renders", () => {
+    const throwing = (src: string) => {
+      if (src.includes("BOOM")) throw new Error("kaboom");
+      return `<p>${src}</p>`;
+    };
+    const html = renderTrackChanges("Good one.\n\nBOOM here.\n", "Good one.\n\nBOOM here.\n", {
+      render: throwing,
+    });
+    expect(html).toContain("cw-error");
+    expect(html).toContain("kaboom");
+    expect(html).toContain("<p>Good one.</p>"); // the healthy block still rendered
+  });
+});