feat: plan 16 P1 (SEC-08/09/19/20/21) — fail-open/DoS/authz hardening

- SEC-08 pii-mask: fail-closed on missing rules / broken regex (no unmasked leak)
- SEC-09: input-size cap + fail-closed reads (security-check/drift-detect/context-compress); non-UTF8 no longer crashes
- SEC-19: control-plane store POSIX flock + atomic tmp+rename write
- SEC-20: new toolchain-verify.sh (missing/PATH-shadowed/in-workspace binary -> refuse); wired into harness-preflight
- SEC-21: model-call timeout 180->60s configurable + per-run call budget
- SEC-11 realized by SEC-17 prod profile (no code)
- 5 fail-able test suites wired into ci-harness-gate.sh; test-integrity manifest regenerated

Verify: SEC+integrity gate 16/0, run-casan4 0-FAIL, adversarial 44/44, no regressions.

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

AINative_OKR_CASAN5/.specify/scripts/bash/harness-preflight.sh

@@ -12,6 +12,16 @@ set -uo pipefail
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROMPT="${1:-}"
 
+# SEC-20 (ARCH-04/09): fail closed if the required toolchain is missing or a
+# required binary is PATH-shadowed by a planted copy (default checks only refuse on
+# missing / in-workspace binaries; prod can set CASAN_TOOLCHAIN_TRUSTED_DIRS).
+if [[ -f "$SCRIPT_DIR/toolchain-verify.sh" ]]; then
+  if ! bash "$SCRIPT_DIR/toolchain-verify.sh" >/dev/null 2>&1; then
+    echo "PREFLIGHT_BLOCK toolchain-verify (missing/shadowed binary)" >&2
+    exit 1
+  fi
+fi
+
 MODEL=""
 while [[ $# -gt 0 ]]; do
   case "$1" in