test(cli): fix flaky rename watch test with deterministic waiter polling

The rename test relied on a fixed 300ms setTimeout to fire after the CLI
subprocess had registered its waiter — adequate in isolation but flaky
under full-suite load on Windows (CLI spawn + first HTTP request can
exceed 300ms). Add a tiny public MailboxStore.waiterCount(name) helper
so the test can poll until the waiter is actually registered before
triggering the rename. Also tighten the missing-name assertion from
not-zero to the contract-exact exit code 1.

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

node/src/db.ts

@@ -294,6 +294,10 @@ export class MailboxStore {
     this.waiters.delete(oldName);
   }
 
+  waiterCount(name: string): number {
+    return this.waiters.get(name)?.size ?? 0;
+  }
+
   rejectAllWaiters(): void {
     for (const bucket of this.waiters.values()) {
       for (const w of bucket) w.resolve({ kind: "aborted" });