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

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

src/store.ts

@@ -61,8 +61,8 @@ export class CoauthorStore {
    * 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). `mutate` MUST be synchronous: the
+   * referenced by no thread, attribution, OR proposal are pruned (threads,
+   * attributions, and proposals all keep their anchors — F4). `mutate` MUST be synchronous: the
    * read-modify-write (and the self-write mark) completes within this call.
    */
   update(docPath: string, mutate: (artifact: Artifact) => void): Artifact {
@@ -71,6 +71,7 @@ export class CoauthorStore {
     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];