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/context-compress.py

@@ -55,11 +55,37 @@ def estimate_tokens(text: str) -> int:
     return len(text.split())
 
 
+# SEC-09 (M-10): bound input size (DoS) and read fail-closed. Non-UTF8 degrades via
+# errors="replace" instead of crashing; oversize/unreadable input exits non-zero and
+# emits nothing (never a crash traceback, never silent truncation).
+MAX_BYTES = int(os.environ.get("CASAN_MAX_INPUT_BYTES", str(2 * 1024 * 1024)))
+
+
+def read_capped(src: str) -> str:
+    try:
+        if src == "-":
+            data = sys.stdin.buffer.read(MAX_BYTES + 1)
+        else:
+            with open(src, "rb") as fh:
+                data = fh.read(MAX_BYTES + 1)
+    except OSError as exc:
+        print(f"COMPRESS_FAIL unreadable_input: {exc}", file=sys.stderr)
+        raise SystemExit(1)
+    if len(data) > MAX_BYTES:
+        print(f"COMPRESS_FAIL input_exceeds_cap({MAX_BYTES}B) fail-closed", file=sys.stderr)
+        raise SystemExit(1)
+    return data.decode("utf-8", errors="replace")
+
+
 def load_patterns(path: str):
     if not path:
         return []
-    with open(path, encoding="utf-8") as fh:
-        return [line.strip() for line in fh if line.strip()]
+    try:
+        with open(path, encoding="utf-8", errors="replace") as fh:
+            return [line.strip() for line in fh if line.strip()]
+    except OSError as exc:
+        print(f"COMPRESS_FAIL must_keep_file_unreadable: {exc}", file=sys.stderr)
+        raise SystemExit(1)
 
 
 def is_must_keep(line: str, patterns) -> bool:
@@ -114,7 +140,7 @@ def main() -> int:
     )
     args = ap.parse_args()
 
-    raw = sys.stdin.read() if args.input == "-" else open(args.input, encoding="utf-8").read()
+    raw = read_capped(args.input)
     must = load_patterns(args.must_keep_file)
 
     if args.failed: