This ticket implements clean shutdown: on the batcher shutdown signal, the dispatcher stops pulling, performs a full drain, and threads are joined in the correct order.

This ticket should include:

• The batcher shutdown signal stopping the dispatcher and issuing a full drain: queues empty, in-flight FIFO empty, lastApplied published to the last dispatched op.
• Join ordering: advancer joined before the workers.
• The drain is the batcher-driven clean-shutdown path. A global-shutdown interrupt already exists and abandons unapplied items for restart recovery to replay; a global interrupt that arrives during the clean shutdown drain still abandons entries.