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

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

src/starkernel/vm/kernel_hermes.c

@@ -47,6 +47,7 @@
 #include <string.h>
 #include "starkernel/vm/kernel_hermes.h"
 #include "starkernel/vm/stadium.h"
+#include "starkernel/q48_16.h"
 
 static SkHermesMessage sk_hermes_msgs[SK_HERMES_MSG_MAX];
 
@@ -181,4 +182,24 @@ int sk_hermes_release(SkHermesMessage *msg) {
     return rc;
 }
 
+int sk_hermes_decay(SkHermesMessage *msg) {
+    uint64_t before, after;
+    StadiumCell *cell;
+
+    if (!msg || !msg->in_use || msg->stadium_cell < 0) return -1;
+
+    cell   = &stadium_cells()[msg->stadium_cell];
+    before = cell->header.heat;
+    after  = (uint64_t)q48_mul((q48_16_t)before, (q48_16_t)SK_HERMES_Q_DECAY);
+
+    cell->header.heat = after;
+
+    /* Task 2.5: the one site where consumed increments (and where held
+     * pays for it). SXXXVII.3: decay's delta is exact and in hand here. */
+    sk_hermes_consumed += before - after;
+    sk_hermes_held     -= before - after;
+
+    return 0;
+}
+
 #endif /* __STARKERNEL__ */