F3 SLICE-3: pending-edit registry + sidecar section-merge update (INV-9 groundwork) (#6)

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

test/store.test.ts

@@ -54,4 +54,38 @@ describe("CoauthorStore", () => {
     expect(second).toBe(first);
     expect(first).toBe(serializeArtifact(a));
   });
+
+  it("update(): two writers merge sections instead of clobbering (F3 §6.2)", () => {
+    const store = new CoauthorStore(root);
+    store.update("d.md", (a) => {
+      a.anchors["a_t"] = { fingerprint: { text: "t", before: "", after: "", lineHint: 0 } };
+      a.threads.push({ id: "t1", anchorId: "a_t", status: "open", messages: [] });
+    });
+    store.update("d.md", (a) => {
+      a.anchors["a_at"] = { fingerprint: { text: "x", before: "", after: "", lineHint: 0 } };
+      a.attributions.push({
+        id: "at1", anchorId: "a_at", author: { kind: "human", id: "ben" },
+        createdAt: "2026-06-10T00:00:00.000Z", updatedAt: "2026-06-10T00:00:00.000Z",
+      });
+    });
+    const loaded = store.load("d.md")!;
+    expect(loaded.threads).toHaveLength(1);
+    expect(loaded.attributions).toHaveLength(1);
+    expect(Object.keys(loaded.anchors).sort()).toEqual(["a_at", "a_t"]);
+  });
+
+  it("update() prunes anchors no longer referenced by threads or attributions", () => {
+    const store = new CoauthorStore(root);
+    store.update("d.md", (a) => {
+      a.anchors["a_old"] = { fingerprint: { text: "o", before: "", after: "", lineHint: 0 } };
+      a.attributions.push({
+        id: "at1", anchorId: "a_old", author: { kind: "human", id: "ben" },
+        createdAt: "2026-06-10T00:00:00.000Z", updatedAt: "2026-06-10T00:00:00.000Z",
+      });
+    });
+    store.update("d.md", (a) => {
+      a.attributions = [];
+    });
+    expect(Object.keys(store.load("d.md")!.anchors)).toEqual([]);
+  });
 });