Flow — Master compositions

Bigger, cross-layer examples: a full Master combining Mailbox, Pool, and
Select/Group in one coordinator.

• Minimal Master
• Master with Pool
• Multi-worker Master
• Pipeline of Masters
• Request-response between Masters
• Pool → Mailbox → Pool roundtrip
• Mixed types through shared mailbox
• Batch receive + pool return
• Pool hooks + mailbox flow
• Close ordering: pool then mailbox
• Close ordering: mailbox then pool
• Pool + Mailbox flow
• Master shutdown: close → stdlib walk → free
• Master batch collect: receive_batch → put_all
• Master pre-shutdown collect