perf(openclaw-tps-mail): measure mailbox-lock contention from the aged-nack sweep and obligation writers #536

Follow-up from cli#534 (#533).

After cli#534 the aged-nack sweep in plugins/openclaw-tps-mail holds the per-mailbox lock (packages/agent/src/lib/mail-lock.ts) through selection, release, record deletion and receipt deletion, and the obligation writers take the same lock. That lock covers the whole mailbox, including ordinary mail operations, and synchronous obligation writers can wait on it. The sweep skips a busy mailbox, so it cannot stall delivery, but nothing measures how long writers wait.

Measure it: record lock wait and hold times for the sweep and the obligation writers on a busy mailbox (a real-file benchmark with concurrent writers), and report p50/p99. If writers wait more than a few milliseconds at p99, narrow the sweep's critical section or move obligations to their own lock with a documented ordering.

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