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/ci-harness-gate.sh

@@ -108,6 +108,10 @@ run "phase-sec10-agent-identity" bash "$TESTS/phase-sec10-tests.sh"
 run "phase-sec13-ssrf"           bash "$TESTS/phase-sec13-tests.sh"
 run "phase-sec27-log-controlchar" bash "$TESTS/phase-sec27-tests.sh"
 run "phase-sec28-path-traversal" bash "$TESTS/phase-sec28-tests.sh"
+run "phase-sec12-drift-invariant" bash "$TESTS/phase-sec12-tests.sh"
+run "phase-sec29-audit-failclosed" bash "$TESTS/phase-sec29-tests.sh"
+run "phase-sec30-approval-replay" bash "$TESTS/phase-sec30-tests.sh"
+run "phase-sec15-low-cluster"    bash "$TESTS/phase-sec15-tests.sh"
 
 # ARCH-02: coverage cannot silently drop; ARCH-01: harness/policy cannot silently
 # drift. Both SKIP cleanly when no manifest is provisioned (non-strict dev/CI).