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/tool-exec.sh

@@ -29,7 +29,13 @@ elif command -v perl >/dev/null 2>&1; then
   perl -e 'my $t=shift; $SIG{ALRM}=sub{exit 124}; alarm($t); exec @ARGV or exit 127;' "$TIMEOUT" "$@"
   rc=$?
 else
-  echo "TOOL_EXEC_NO_TIMEOUT_BACKEND" >&2
+  # SEC-15: no timeout backend. In enforced mode REFUSE (fail-closed) rather than
+  # run a command that could hang the pipeline; dev warns and proceeds.
+  if [[ "${CASAN_PROFILE:-}" == "prod" || "${CASAN_TOOL_EXEC_STRICT:-}" == "1" ]]; then
+    echo "TOOL_EXEC_NO_TIMEOUT_BACKEND refusing (fail-closed: install coreutils timeout or perl)" >&2
+    exit 2
+  fi
+  echo "TOOL_EXEC_NO_TIMEOUT_BACKEND (dev: running without timeout)" >&2
   "$@"
   rc=$?
 fi