feat: CoeditingRegistry opt-in gate (INV-10, spec §6.4) — enter/stop commands + context keys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/coeditingRegistry.ts

@@ -0,0 +1,57 @@
+/**
+ * CoeditingRegistry — the INV-10 activation gate (spec §6.4). Holds the set of
+ * documents the writer has explicitly entered into coediting; every surface
+ * (SCM/QuickDiff, comments, preview annotations, edit commands, decorations)
+ * checks membership before attaching. Persisted in workspaceState so a reload
+ * restores the set (the durable sidecar carries the artifacts; this carries
+ * only the mode).
+ */
+import * as vscode from "vscode";
+
+const STATE_KEY = "cowriting.coeditingSet";
+
+export class CoeditingRegistry implements vscode.Disposable {
+  private readonly set: Set<string>;
+  private readonly emitter = new vscode.EventEmitter<{ uri: string; coediting: boolean }>();
+  readonly onDidChange = this.emitter.event;
+  /** Set by the SCM surface (Task 3) so syncContext can expose the baseline mode. */
+  baselineModeOf: ((uri: vscode.Uri) => "head" | "snapshot" | undefined) | undefined;
+
+  constructor(private readonly state: vscode.Memento) {
+    this.set = new Set(state.get<string[]>(STATE_KEY, []));
+  }
+
+  enter(uri: vscode.Uri): void {
+    const key = uri.toString();
+    if (this.set.has(key)) return;
+    this.set.add(key);
+    void this.state.update(STATE_KEY, [...this.set]);
+    this.emitter.fire({ uri: key, coediting: true });
+  }
+
+  exit(uri: vscode.Uri): void {
+    const key = uri.toString();
+    if (!this.set.delete(key)) return;
+    void this.state.update(STATE_KEY, [...this.set]);
+    this.emitter.fire({ uri: key, coediting: false });
+  }
+
+  isCoediting(uri: vscode.Uri): boolean {
+    return this.set.has(uri.toString());
+  }
+
+  list(): string[] {
+    return [...this.set];
+  }
+
+  syncContext(editor: vscode.TextEditor | undefined): void {
+    const coediting = !!editor && this.isCoediting(editor.document.uri);
+    void vscode.commands.executeCommand("setContext", "cowriting.isCoediting", coediting);
+    const mode = coediting && editor ? this.baselineModeOf?.(editor.document.uri) : undefined;
+    void vscode.commands.executeCommand("setContext", "cowriting.baselineMode", mode ?? "");
+  }
+
+  dispose(): void {
+    this.emitter.dispose();
+  }
+}