F2 SLICE-3: ThreadController on the Comments API + thread mutations (#4)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/threadModel.ts

@@ -0,0 +1,48 @@
+/**
+ * vscode-free mutations on the Artifact (spec §6.5). Keeping thread logic here
+ * (not in the vscode controller) keeps it unit-testable; ThreadController is a
+ * thin adapter that calls these and persists via CoauthorStore.
+ */
+import {
+  newId,
+  type Artifact,
+  type Fingerprint,
+  type Provenance,
+  type ThreadStatus,
+} from "./model";
+
+export interface NewMessage {
+  author: Provenance;
+  body: string;
+}
+
+function message(m: NewMessage) {
+  return { id: newId("m"), author: m.author, body: m.body, createdAt: new Date().toISOString() };
+}
+
+/** Lazily-created anchor + thread for a new region discussion (PUC-1). */
+export function addThread(
+  artifact: Artifact,
+  fingerprint: Fingerprint,
+  first: NewMessage,
+): { threadId: string; anchorId: string } {
+  const anchorId = newId("a");
+  const threadId = newId("t");
+  artifact.anchors[anchorId] = { fingerprint };
+  artifact.threads.push({ id: threadId, anchorId, status: "open", messages: [message(first)] });
+  return { threadId, anchorId };
+}
+
+/** Append a reply (PUC-2). Throws if the thread is unknown. */
+export function appendMessage(artifact: Artifact, threadId: string, m: NewMessage): void {
+  const t = artifact.threads.find((x) => x.id === threadId);
+  if (!t) throw new Error(`unknown thread id: ${threadId}`);
+  t.messages.push(message(m));
+}
+
+/** Resolve/reopen a thread (PUC-2). Throws if the thread is unknown. */
+export function setStatus(artifact: Artifact, threadId: string, status: ThreadStatus): void {
+  const t = artifact.threads.find((x) => x.id === threadId);
+  if (!t) throw new Error(`unknown thread id: ${threadId}`);
+  t.status = status;
+}