docs(db): mark waiterCount as @internal test helper

Final review flagged that waiterCount looks like a public API but is
only called from the rename-watch flake fix test. The behavior is fine
to keep public (in-memory, safe), but the docstring now matches the
intent so future API-surface scans don't load-bear on it.

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

node/src/db.ts

@@ -294,6 +294,7 @@ export class MailboxStore {
     this.waiters.delete(oldName);
   }
 
+  /** @internal Test helper to wait for a long-poll waiter to register. Not part of the public contract. */
   waiterCount(name: string): number {
     return this.waiters.get(name)?.size ?? 0;
   }