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>

include/starkernel/vm/stadium.h

@@ -622,6 +622,18 @@ int stadium_cell_decay(size_t cell_index, uint64_t factor_q48, uint64_t *consume
  * session_set_pinned(), the sole pin choke point). -1 if out of range. */
 int stadium_cell_set_pin(size_t cell_index, int pinned);
 
+/*
+ * FABRIC-3.10.8 K-4: quota slots are released (cells to the best donor,
+ * ruled 2026-09-26). stadium_release_quota() retires vm_id's slot (every
+ * patron but its pending messages evicted); stadium_retire_sweep() releases
+ * each retiring slot once nothing is resident. Call release on VM death
+ * (KILL, reap) and when a self-test's synthetic id is done. Hera's slot is
+ * never released. Returns 0, or -1 (no such slot, already retiring, Hera).
+ */
+int      stadium_release_quota(VMUuid vm_id);
+void     stadium_retire_sweep(void);
+uint64_t stadium_slots_released(void);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_STADIUM_H */