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/kernel_main.c

@@ -914,7 +914,45 @@ static void kernel_main_deep(BootInfo *boot_info) {
              * hand): held == pulled - returned - consumed, at rest. */
             if (held2 != pulled2 - returned2 - consumed2) ok = 0;
 
+            /* Task 2.5: second cycle, with decay. Allocate to exhaustion
+             * again, decay every message once, and check `consumed`
+             * grew by EXACTLY the sum of (heat_before - heat_after)
+             * measured independently from the Stadium cells, each
+             * message's new heat is q48_mul(before, Q_DECAY), and the
+             * audit invariant still holds mid-hold. Then release and
+             * confirm the reservoir returns reservoir0 minus exactly what
+             * was consumed (decayed heat does not return, SXL.4). */
+            uint64_t decay_expected = 0;
+            uint64_t held3, pulled3, returned3, consumed3;
+            uint64_t held4, pulled4, returned4, consumed4;
+            uint64_t n2 = 0;
+            while (n2 < SK_HERMES_MSG_MAX && sk_hermes_alloc(alloc_test_id, &msg) == 0) {
+                msgs[n2] = msg;
+                n2++;
+            }
+            if (n2 != expected_n) ok = 0;
+            for (i = 0; i < n2; i++) {
+                uint64_t before = stadium_cells()[msgs[i]->stadium_cell].header.heat;
+                uint64_t want   = (uint64_t)q48_mul((q48_16_t)before, (q48_16_t)SK_HERMES_Q_DECAY);
+                if (sk_hermes_decay(msgs[i]) != 0) ok = 0;
+                if (stadium_cells()[msgs[i]->stadium_cell].header.heat != want) ok = 0;
+                decay_expected += before - want;
+            }
+            sk_hermes_ledger(&held3, &pulled3, &returned3, &consumed3);
+            if (decay_expected == 0) ok = 0; /* vacuity guard: decay must bite */
+            if (consumed3 - consumed2 != decay_expected) ok = 0;
+            if (held3 != pulled3 - returned3 - consumed3) ok = 0;
+            for (i = 0; i < n2; i++) {
+                if (sk_hermes_release(msgs[i]) != 0) ok = 0;
+            }
+            sk_hermes_ledger(&held4, &pulled4, &returned4, &consumed4);
+            if (held4 != held0) ok = 0;
+            if (consumed4 != consumed3) ok = 0;
+            if (held4 != pulled4 - returned4 - consumed4) ok = 0;
+            if (stadium_reservoir_peek(alloc_test_id) != reservoir0 - decay_expected) ok = 0;
+
             console_println(ok ? "PASS" : "FAIL");
+            print_uint("  decay_consumed=", decay_expected);
             print_uint("  reservoir0=", reservoir0);
             print_uint("  Q_SLOT=", SK_HERMES_Q_SLOT);
             print_uint("  expected_n=", expected_n);

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__ */