test(db): add abort-racing-send + rejectAllWaiters coverage; document notifyOneWaiter invariant

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

node/src/db.ts

@@ -274,6 +274,7 @@ export class MailboxStore {
     });
   }
 
+  // Invariant: synchronous from consumeOne to resolve. Introducing an `await` between them risks marking a message delivered with no listener to receive it.
   private notifyOneWaiter(name: string): void {
     const bucket = this.waiters.get(name);
     if (!bucket || bucket.size === 0) return;