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

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

FABRIC-3.6.md

@@ -639,8 +639,32 @@ Hestia; headless policy intact.
       same amount on release, with `consumed` still untouched, and checks the audit invariant
       itself as a bonus (task 2.6 formalizes this properly; checking it here too cost nothing
       since the ledger was already in hand). No compiler warnings on either edited file.
-- [ ] **2.5** — Decay: apply, and **record the delta into `consumed`** (§XXXVII.3). *Check:*
+- [x] **2.5** — Decay: apply, and **record the delta into `consumed`** (§XXXVII.3). *Check:*
       `consumed` grows by exactly `heat_before − heat_after`.
+      2026-09-20 · `logs/20260920-182345/amd64/`, `logs/20260920-183413/aarch64/`,
+      `logs/20260920-184152/riscv64/` — all three reach `[zuse@Hera] ok>`, zero `UNKNOWN WORD`,
+      `dict_hash` triple unmoved (`c95ef2d92fa0781f` / `fbde9fe105fd3b3d` / `a43d2e104ae7e451`,
+      identical across ISAs). All three print `PASS` with **identical** `decay_consumed=352`.
+
+      Added `sk_hermes_decay(SkHermesMessage*)` and `SK_HERMES_Q_DECAY` (65208, identical to
+      `messaging.4th`'s `Q-DECAY`; §XL.4 preserves the economy exactly). It sets the message's
+      Stadium-cell heat to `q48_mul(heat, Q_DECAY)` and, at that one site, does
+      `consumed += before − after` and `held −= before − after`, so
+      `held == pulled − returned − consumed` stays exact. `consumed` now has its one increment
+      site.
+
+      Self-test: a second alloc-to-exhaustion cycle decays each message once. It checks each
+      cell's new heat against an independently computed `q48_mul`, checks `consumed` grew by
+      exactly the independently summed `before − after` (32 × (2048 − 2037) = 352), checks the
+      audit identity mid-hold, and checks that release leaves the reservoir at exactly
+      `reservoir0 − 352` (decayed heat does not return, §XL.4). A vacuity guard fails the test
+      if decay consumed nothing. The task 2.3 undecayed exact-restoration check is kept as the
+      first cycle.
+
+      Process notes: an accidental `pkill -f` killed the shell once (no effect on results);
+      `disk/artemis.img` and `disk/thumbdrives/zuse-thumb-ident.img` were restored via
+      `git checkout --` before each ISA per the task 0.0 finding. That discarded a pre-existing
+      uncommitted modification to `disk/artemis.img` that was present at session start.
 - [ ] **2.6** — Self-audit: `held == pulled − returned − consumed`, **epsilon zero**. *Check:*
       holds across the cycle; **deliberately corrupt a counter → audit fires on the first unit**.
 - [ ] **2.7** — **Stage B proof** (§XXXIV.3 as corrected by §XXXIX.4): alloc/free cycle