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>

src/store.ts

@@ -6,7 +6,7 @@
  */
 import * as fs from "node:fs";
 import * as path from "node:path";
-import { serializeArtifact, type Artifact } from "./model";
+import { emptyArtifact, serializeArtifact, type Artifact } from "./model";
 
 export class CoauthorStore {
   /** @param rootDir absolute workspace-folder root that owns the `.threads/` tree. */
@@ -28,4 +28,25 @@ export class CoauthorStore {
     fs.mkdirSync(path.dirname(p), { recursive: true });
     fs.writeFileSync(p, serializeArtifact(artifact), "utf8");
   }
+
+  /**
+   * Read-modify-write for sidecar co-ownership (spec F3 §6.2): each controller
+   * mutates only its own section against a FRESH load, so ThreadController and
+   * AttributionController never clobber each other. After the mutation, anchors
+   * referenced by neither threads nor attributions are pruned (proposals are
+   * still empty in F3 — revisit in F4).
+   */
+  update(docPath: string, mutate: (artifact: Artifact) => void): Artifact {
+    const artifact = this.load(docPath) ?? emptyArtifact(docPath);
+    mutate(artifact);
+    const referenced = new Set<string>([
+      ...artifact.threads.map((t) => t.anchorId),
+      ...artifact.attributions.map((a) => a.anchorId),
+    ]);
+    for (const id of Object.keys(artifact.anchors)) {
+      if (!referenced.has(id)) delete artifact.anchors[id];
+    }
+    this.save(docPath, artifact);
+    return artifact;
+  }
 }