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/drift-detect.sh

@@ -22,6 +22,7 @@ python - "$GOLDEN" "$CANDIDATE" "$REPORT" <<'PY'
 import difflib
 import hashlib
 import json
+import os
 import pathlib
 import sys
 from datetime import datetime, timezone
@@ -30,8 +31,41 @@ golden_path = pathlib.Path(sys.argv[1])
 candidate_path = pathlib.Path(sys.argv[2])
 report_path = pathlib.Path(sys.argv[3])
 
-golden = golden_path.read_text(encoding="utf-8")
-candidate = candidate_path.read_text(encoding="utf-8")
+# SEC-09 (M-10): cap input size (SequenceMatcher is O(n^2) → DoS) and read
+# fail-closed. A missing/oversize/undecodable input yields a BLOCK verdict, never
+# a crash/traceback (which under set -e would abort ambiguously) and never a silent
+# pass.
+MAX_BYTES = int(os.environ.get("CASAN_MAX_INPUT_BYTES", str(2 * 1024 * 1024)))
+
+
+def block(reason):
+    report_path.parent.mkdir(parents=True, exist_ok=True)
+    report = {
+        "timestamp": datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ"),
+        "harness": "L5-drift-detection", "status": "fail", "action": "block",
+        "reason": reason, "golden_file": str(golden_path), "candidate_file": str(candidate_path),
+    }
+    report_path.write_text(json.dumps(report, indent=2) + "\n", encoding="utf-8")
+    print(f"DRIFT_FAIL reason={reason} report={report_path}")
+    raise SystemExit(2)
+
+
+def read_capped(path):
+    try:
+        size = path.stat().st_size
+    except OSError:
+        block(f"unreadable:{path.name}")
+    if size > MAX_BYTES:
+        block(f"oversize:{path.name}({size}>{MAX_BYTES})")
+    try:
+        # errors="replace" so non-UTF8 bytes degrade to a marker instead of crashing.
+        return path.read_text(encoding="utf-8", errors="replace")
+    except OSError:
+        block(f"unreadable:{path.name}")
+
+
+golden = read_capped(golden_path)
+candidate = read_capped(candidate_path)
 
 similarity = difflib.SequenceMatcher(None, golden, candidate).ratio()
 length_delta = abs(len(candidate) - len(golden)) / max(len(golden), 1)