Add sk_hermes_decay(), ledgering decay into consumed -- FABRIC-3.6.md task 2.5

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

include/starkernel/vm/kernel_hermes.h

@@ -203,8 +203,8 @@ int sk_hermes_release(SkHermesMessage *msg);
  *   held      -= returned amount -- sk_hermes_release(), on success only
  *   pulled    += pulled amount   -- sk_hermes_alloc(), same site as held's
  *   returned  += returned amount -- sk_hermes_release(), same site as held's
- *   consumed  += decay delta     -- task 2.5, not yet wired; reads 0 until
- *                                    then
+ *   held      -= decay delta     -- sk_hermes_decay(), same site as consumed's
+ *   consumed  += decay delta     -- sk_hermes_decay() (task 2.5)
  *
  * The audit invariant this ledger exists to make checkable (task 2.6,
  * epsilon zero): held == pulled - returned - consumed. All four are
@@ -218,10 +218,32 @@ int sk_hermes_release(SkHermesMessage *msg);
  * @param returned  Set to the cumulative total ever returned (may be
  *                  NULL).
  * @param consumed  Set to the cumulative total ever consumed by decay
- *                  (may be NULL) -- always 0 until task 2.5.
+ *                  (may be NULL).
  */
 void sk_hermes_ledger(uint64_t *held, uint64_t *pulled, uint64_t *returned, uint64_t *consumed);
 
+/*
+ * SK_HERMES_Q_DECAY - per-application decay factor, Q48.16. Identical to
+ * messaging.4th's `65208 CONSTANT Q-DECAY` (65208/65536 ~= 0.99499):
+ * FABRIC-3.5.md SXL.4 rules that the consumption economy is preserved
+ * exactly, not re-tuned.
+ */
+#define SK_HERMES_Q_DECAY ((uint64_t)65208)
+
+/*
+ * sk_hermes_decay - Apply one decay step to a live message (task 2.5,
+ * item 28), mirroring MSG-COOL-ONE (`MSG-HEAT@ Q-DECAY Q.* MSG-HEAT!`):
+ * the message's Stadium-cell heat becomes q48_mul(heat, Q_DECAY). Unlike
+ * MSG-COOL-ONE, the destroyed difference is RECORDED (SXXXVII.3): the
+ * delta heat_before - heat_after is added to `consumed` and subtracted
+ * from `held`, so held == pulled - returned - consumed stays exact.
+ *
+ * @param msg  A live slot from sk_hermes_alloc(). Refused (returns -1, no
+ *             effect) if NULL, not in use, or holding no Stadium cell.
+ * @return 0 on success, -1 if refused.
+ */
+int sk_hermes_decay(SkHermesMessage *msg);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_KERNEL_HERMES_H */