Stop mailbox-mail occupancy from flooding the send queue

[TheGreatAxios]

Summary

• Occupancy holds one in-flight mailbox-mail drive so store-subscribe, stall-poll, and idle-with-fleet settle cannot each call send and fill the depth-16 queue.
• After a snapshot, those worker ids stay in-flight until take on success or release on failed send so a later flush can retry.
• Wake copy says occupancy delivered the reports; it does not say already collected on first delivery.

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes (6848 pass, 0 fail)

Fixes CL-7739
Fixes CL-7699

[linear-code]

CL-7739

CL-7699

[TheGreatAxios]

Independent critic (CL-7739 + CL-7699)

Meets both tickets: occupancy holds one in-flight latchMailboxMailDrive; overlapping flushes do not double-send; a no-op does not latch; a new terminal can send after settle; sendQueueMax stays 16. Terminals are taken/latched before a second parent turn; failed send stays waitable; wake copy is occupancy-delivered, not “already collected”.

Blocking: none. Merge.

Should-fix (non-blocking): CHANGELOG puts the occupancy note under already-shipped ## [0.3.21]. This did not ship in 0.3.21. Move to a new [Unreleased] block.

Cannot GitHub-approve (same author). CI green.