feat(f8): SidecarStore interface + GlobalSidecarStore (sha256 key, untitled in-memory)

Spec §6.2/§6.4: out-of-workspace file: → <globalStorage>/sidecars/<sha256(uri)>.json
(INV-19/24); untitled: → in-memory only (F6 degrade). update() carries the INV-16
newer-major throw + anchor prune; consumeSelfWrite is a no-op (outside the .threads
watcher). Mirrors src/baselineStore.ts; vscode-free, unit-tested.

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

src/globalSidecarStore.ts

@@ -0,0 +1,89 @@
+/**
+ * GlobalSidecarStore — the out-of-workspace/untitled authoring sidecar (F8 spec
+ * §6.2/§6.4), mirroring BaselineStore (src/baselineStore.ts): vscode-free (Node
+ * fs/crypto only), one `Artifact` JSON per document in VS Code's per-extension
+ * GLOBAL storage, NEVER the repo (INV-19/24). The key passed in is the DOCUMENT
+ * KEY (the URI string for the docs this store serves):
+ *   - `file:` URI  → disk at `<dir>/sidecars/<sha256(uri)>.json`.
+ *   - `untitled:`  → an in-memory Map only (no durable identity — the F6
+ *     degrade); lost on reload, and never read by mergeArtifacts (INV-25).
+ * `consumeSelfWrite` is a no-op (these artifacts are outside the `.threads/`
+ * watcher, so there is no self-write storm to suppress).
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { createHash } from "node:crypto";
+import {
+  SCHEMA_VERSION,
+  emptyArtifact,
+  isNewerMajor,
+  serializeArtifact,
+  type Artifact,
+} from "./model";
+import type { SidecarStore } from "./sidecarStore";
+
+export class GlobalSidecarStore implements SidecarStore {
+  /** untitled keys live here only — no durable identity to persist against. */
+  private readonly memory = new Map<string, Artifact>();
+
+  /** @param storageDir absolute VS Code GLOBAL-storage dir (context.globalStorageUri.fsPath). */
+  constructor(private readonly storageDir: string) {}
+
+  private isUntitled(key: string): boolean {
+    return key.startsWith("untitled:");
+  }
+
+  /** Filesystem-safe storage key for a persistable (file:) doc: sha256 of its URI. */
+  private storageKey(key: string): string {
+    return createHash("sha256").update(key).digest("hex");
+  }
+
+  /** Disk path for a `file:` key, or undefined for an in-memory untitled key. */
+  sidecarPath(key: string): string | undefined {
+    if (this.isUntitled(key)) return undefined;
+    return path.join(this.storageDir, "sidecars", `${this.storageKey(key)}.json`);
+  }
+
+  load(key: string): Artifact | null {
+    if (this.isUntitled(key)) return this.memory.get(key) ?? null;
+    const p = this.sidecarPath(key)!;
+    if (!fs.existsSync(p)) return null;
+    return JSON.parse(fs.readFileSync(p, "utf8")) as Artifact;
+  }
+
+  save(key: string, artifact: Artifact): void {
+    if (this.isUntitled(key)) {
+      this.memory.set(key, artifact);
+      return;
+    }
+    const p = this.sidecarPath(key)!;
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    fs.writeFileSync(p, serializeArtifact(artifact), "utf8");
+  }
+
+  /** See CoauthorStore.update — same INV-16 throw + anchor prune, no self-write mark. */
+  update(key: string, mutate: (artifact: Artifact) => void): Artifact {
+    const artifact = this.load(key) ?? emptyArtifact(key);
+    if (isNewerMajor(artifact)) {
+      throw new Error(
+        `refusing to write ${key}: sidecar schemaVersion ${artifact.schemaVersion} > supported ${SCHEMA_VERSION} (INV-16)`,
+      );
+    }
+    mutate(artifact);
+    const referenced = new Set<string>([
+      ...artifact.threads.map((t) => t.anchorId),
+      ...artifact.attributions.map((a) => a.anchorId),
+      ...artifact.proposals.map((p) => p.anchorId),
+    ]);
+    for (const id of Object.keys(artifact.anchors)) {
+      if (!referenced.has(id)) delete artifact.anchors[id];
+    }
+    this.save(key, artifact);
+    return artifact;
+  }
+
+  /** No-op: global artifacts are outside the `.threads/` watcher. */
+  consumeSelfWrite(_fsPath: string): boolean {
+    return false;
+  }
+}