Add diagnostic scan cross-check of the Hermes counters -- FABRIC-3.6.md task 2.8

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

FABRIC-3.6.md

@@ -712,8 +712,25 @@ Hestia; headless policy intact.
 
       **Phase 2 gate: passed** (2.7 holds on all three arches). Remaining Phase 2 work is 2.8
       (diagnostic scan cross-check).
-- [ ] **2.8** — Scan-based cross-check of the counters, diagnostics only, off the hot path
+- [x] **2.8** — Scan-based cross-check of the counters, diagnostics only, off the hot path
       (§XXXVII.4). *Check:* scan agrees with counters.
+      2026-09-21 · `logs/20260921-083638/amd64/`, `logs/20260921-083950/aarch64/`,
+      `logs/20260921-084314/riscv64/` — all print `Scan cross-check (counters vs arena): PASS`
+      with identical `scan_held_before_decay=8192`, `scan_held_after_decay=8148`; Stage B and
+      the main self-test still `PASS`, `audit_failures=0`, zero `UNKNOWN WORD`, `dict_hash`
+      triple unmoved and identical across ISAs.
+
+      Added `sk_hermes_scan_held()` (walks the arena, sums live Stadium-cell heat of in-use
+      messages, reports the live count) and `sk_hermes_scan_check()` (scan == `held`, exact).
+      Called only from the self-test, never from alloc/release/decay. The check runs at rest
+      (0 live), mid-hold (4 live, 4×2048 = 8192), after decay (8148 = 8192 − 4×11) and after
+      release (0), with a vacuity guard (non-zero mid-hold, strictly less after decay).
+
+      Limit, stated: the scan reads the same Stadium cells the counters were derived from, so
+      it verifies the counters against the arena, not the Stadium cells against ground truth;
+      the latter is `stadium_conserved()`'s job (2.7).
+
+      **Phase 2 is complete (2.1–2.8).** Phase 3 remains blocked on B1, B2, B4.
 
 ## Phase 3 — Cutover — **BLOCKED**