Stage K-4: quota slots released to the best donor; message cells out of the admission fallback (FABRIC-3.10.8 §4.4)

- stadium_release_quota(): evict every patron but pending messages, mark
  the slot retiring; stadium_retire_sweep() releases a retiring slot once
  nothing is resident -- free cells to the best donor (ruled), word row
  and block-table entries cleared, slot free for reuse. Swept at release
  and after evictions while any slot is retiring.
- Called on KILL and reap, and for the nine synthetic self-test ids after
  the boot self-tests. Hera's slot never released.
- Admission fallback skips DELIVER (message) cells.
- Probe (removed): slots in use == live VMs after boot and after 56-64
  churned VMs on all three ISAs; forced fallback kept the message cell
  resident and intact; conserved everywhere; K-1's post-churn Hermes
  refusal gone (1 2 + . -> 3 on the churned system). Heap growth per
  28-VM churn halved (~3.4 KB/VM still retained -- open as K-8).
- Standard sequence on amd64/aarch64/riscv64: hash, G, ledger unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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