F4 SLICE-1: anchor prune counts proposal anchorIds (closes store.ts F4 TODO) (#12)

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

test/proposalModel.test.ts

@@ -1,3 +1,6 @@
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import { describe, expect, it } from "vitest";
 import {
   emptyArtifact,
@@ -5,6 +8,7 @@ import {
   type Artifact,
   type Proposal,
 } from "../src/model";
+import { CoauthorStore } from "../src/store";
 
 const agent = {
   kind: "agent" as const,
@@ -44,3 +48,28 @@ describe("Proposal model (spec §6.3, INV-13)", () => {
     expect(serializeArtifact(again)).toBe(serializeArtifact(a));
   });
 });
+
+describe("CoauthorStore prune with proposals (spec §6.3)", () => {
+  it("retains anchors referenced only by proposals; prunes them after removal", () => {
+    const dir = mkdtempSync(join(tmpdir(), "cowriting-prune-"));
+    try {
+      const store = new CoauthorStore(dir);
+      store.update("docs/x.md", (a) => {
+        a.anchors["a_p"] = {
+          fingerprint: { text: "t", before: "", after: "", lineHint: 0 },
+        };
+        a.proposals.push({
+          id: "pr_1", anchorId: "a_p", replacement: "r", author: agent,
+          createdAt: "2026-06-10T00:00:00.000Z",
+        });
+      });
+      expect(store.load("docs/x.md")!.anchors["a_p"]).toBeDefined();
+      store.update("docs/x.md", (a) => {
+        a.proposals = a.proposals.filter((p) => p.id !== "pr_1");
+      });
+      expect(store.load("docs/x.md")!.anchors["a_p"]).toBeUndefined();
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});