feat(dqn-v2): A.2 ISV layout fingerprint at ISV[37..39) (tail placement)

Implements spec §4.A.2 structural layout fingerprint with tail placement
rather than head placement (spec alternative: §4.A.2 Step 5.3 alt).

Head placement (ISV[0..2)) was rejected because isv_signals[0] and [1]
are actively written by the isv_signal_update kernel (Q-drift EMA and
gradient-norm EMA). Shifting those would require updating every literal
reference in experience_kernels.cu — a larger change than warranted for
pure contract enforcement. Tail placement leaves all existing indices
intact, touches zero kernel .cu files, and fulfils the same design contract.

Key changes:
- ISV_LAYOUT_FINGERPRINT_LO_INDEX = 37, HI_INDEX = 38 (u64 across 2×f32).
- LAYOUT_FINGERPRINT_CURRENT: u64 = FNV-1a of slot-list seed bytes.
  Value: 0x85d4d76b578a7c17. Any slot change updates seed bytes,
  which updates the hash automatically.
- Constructor writes fingerprint after zero-init; calls
  check_layout_fingerprint() to self-verify before returning.
- check_layout_fingerprint(): reads pinned slots [37..39), recomposes u64,
  fails-fast on mismatch with "retrain required" message.
- Error message does NOT mention migration as an option.
- Pre-commit hook rejects `fn migrate_isv|upgrade_isv` names — makes the
  no-migration rule structurally enforced (check_no_isv_migrations).
- ISV_TOTAL_DIM: 37 → 39.
- Zero existing index shifts (no kernel literal sites affected).
- StateResetRegistry entry renamed ISV_SCHEMA_VERSION → ISV_LAYOUT_FINGERPRINT.
- ResetCategory::SchemaContract docstring updated to remove "migration" framing.
- docs/isv-slots.md: updated table + design note for tail placement.

Tests: state_reset_registry 3 unit tests pass with renamed entry.
       cargo check -p ml clean (pre-existing warnings only).

Plan 1 Task 5. Spec §4.A.2 (tail-placement alternative).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/pre-commit-hook.sh

@@ -105,8 +105,23 @@ check_no_todo_fixme() {
     fi
 }
 
+# DQN v2 Invariant (spec §4.A.2): no ISV migration functions allowed.
+# The layout fingerprint is fail-fast only; no migration path exists.
+check_no_isv_migrations() {
+    local bad=$(git diff --cached -U0 -- crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs \
+        crates/ml/src/trainers/dqn/trainer/ 2>/dev/null | \
+        grep -E '^\+.*fn\s+(migrate|upgrade)_isv' || true)
+    if [ -n "$bad" ]; then
+        echo "❌ Spec §4.A.2 violation: ISV migration functions are forbidden"
+        echo "   (layout fingerprint is fail-fast only; no migration path exists)"
+        echo "$bad" | sed 's/^/     /'
+        return 1
+    fi
+}
+
 check_audit_doc_updates || exit 1
 check_no_todo_fixme || exit 1
+check_no_isv_migrations || exit 1
 
 echo "✅ All pre-commit checks passed!"
 echo ""