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/sidecarStore.ts

@@ -0,0 +1,35 @@
+/**
+ * SidecarStore — the storage surface the three authoring controllers depend on
+ * (F8 spec §6.2/§6.4). One per-document `Artifact` keyed by a string `key` (the
+ * document key — repo-relative path in-workspace, URI string otherwise; the
+ * router's keyOf is the single source). Two implementations:
+ *   - CoauthorStore (src/store.ts) — the committable repo `.threads/` sidecar,
+ *     keyed by repo-relative path (INV-2, byte-for-byte unchanged; conforms
+ *     structurally — not modified for F8).
+ *   - GlobalSidecarStore (src/globalSidecarStore.ts) — out-of-workspace `file:`
+ *     and `untitled:` docs, in VS Code GLOBAL storage keyed by sha256(uri)
+ *     (INV-19/24/25; untitled in-memory only).
+ * SidecarRouter (src/sidecarRouter.ts) implements this AND adds keyOf/sidecarPath;
+ * the controllers receive the router.
+ */
+import type { Artifact } from "./model";
+
+export interface SidecarStore {
+  /** Load the artifact for `key`, or null if none persisted. */
+  load(key: string): Artifact | null;
+  /** Overwrite the artifact for `key` (newest wins, no history). */
+  save(key: string, artifact: Artifact): void;
+  /**
+   * Read-modify-write: load (or empty), apply `mutate`, prune anchors referenced
+   * by no thread/attribution/proposal, persist, return the result. MUST be
+   * synchronous (the F3 co-ownership contract). Throws on a newer-major sidecar
+   * (INV-16 backstop).
+   */
+  update(key: string, mutate: (artifact: Artifact) => void): Artifact;
+  /**
+   * Decrement-and-report whether `fsPath` is a sidecar this store just wrote
+   * (repo sidecars only; the global store is outside the `.threads/` watcher so
+   * its impl is a no-op returning false).
+   */
+  consumeSelfWrite(fsPath: string): boolean;
+}