feat: plan 16 P2 batch 2 (SEC-12 drift, SEC-29 audit fail-closed, SEC-30 replay, SEC-15 low)

- SEC-12: drift-detect adds semantic invariants — negation-flip detection (a dropped
  "not" now FAILS despite high char-similarity) + env must-keep patterns.
- SEC-29 (X-05): governance-check audit write fails CLOSED — an unwritable audit log
  denies the action and empties the output (no unaudited output).
- SEC-30 (X-06): approval-verify records a one-time-use nonce (sha of token/sig) and
  rejects replays (enforced mode / when a nonce ledger is set); dev unchanged.
- SEC-15 (low): typosquat distance<=2 with the levenshtein length-sentinel bug fixed
  (no false positives); tool-exec fails closed with no timeout backend in enforced
  mode; validate-tool-input now validates nested objects/arrays recursively.

Verify: new SEC suites all green via gate, run-casan4 0-FAIL, adversarial 44/44,
track-c 29/0, h5-approval 12/0, no regressions.

Plan-16 P2 remaining: infra-gated only (SEC-14/22/23/24/25/26).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

AINative_OKR_CASAN5/.specify/scripts/bash/drift-detect.sh

@@ -72,18 +72,52 @@ length_delta = abs(len(candidate) - len(golden)) / max(len(golden), 1)
 
 status = "pass"
 action = "allow"
+reasons = []
 if similarity < 0.70 or length_delta > 0.50:
     status = "fail"
     action = "block_or_fallback"
+    reasons.append("low_similarity_or_length_delta")
 elif similarity < 0.85 or length_delta > 0.30:
     status = "warn"
     action = "require_review"
 
+# SEC-12: char-similarity alone misses SEMANTIC inversion — dropping a negation
+# ("must NOT deploy" -> "must deploy") keeps similarity high but flips meaning. A
+# candidate that removes negation tokens present in the golden is treated as drift.
+import re as _re
+NEG = _re.compile(
+    r"\b(not|no|never|cannot|can't|don't|must not|mustn't|deny|denied|reject|disable|"
+    r"disabled|forbid|prohibit|block|blocked|không|đừng|cấm|từ chối)\b",
+    _re.IGNORECASE,
+)
+golden_neg = len(NEG.findall(golden))
+cand_neg = len(NEG.findall(candidate))
+if golden_neg > cand_neg:
+    # The dangerous case: looks nearly identical but a negation vanished.
+    status, action = "fail", "block_or_fallback"
+    reasons.append(f"negation_dropped(golden={golden_neg},candidate={cand_neg})")
+
+# must-keep invariants: regex patterns that MUST still appear in the candidate.
+_mk = os.environ.get("CASAN_DRIFT_MUSTKEEP_FILE", "")
+missing = []
+if _mk and os.path.isfile(_mk):
+    for _line in open(_mk, encoding="utf-8", errors="replace"):
+        _pat = _line.strip()
+        if _pat and not _re.search(_pat, candidate):
+            missing.append(_pat)
+    if missing:
+        status, action = "fail", "block_or_fallback"
+        reasons.append(f"must_keep_missing={len(missing)}")
+
 report = {
     "timestamp": datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ"),
     "harness": "L5-drift-detection",
     "status": status,
     "action": action,
+    "reasons": reasons,
+    "golden_negations": golden_neg,
+    "candidate_negations": cand_neg,
+    "must_keep_missing": missing,
     "similarity_ratio": round(similarity, 4),
     "length_delta_ratio": round(length_delta, 4),
     "golden_hash": hashlib.sha256(golden.encode()).hexdigest(),