Implement USER-WP-0024 security layer conformance

Declare Engine/PIP machine-readably, publish a total fail-closed PEP
stance map, stop minting local decision ids on engine-unavailable DENY,
bind allows to a 30s request lifetime, confine the local authorization
double, classify evidence and emit a denial/revocation heartbeat, and
prove access-control facts remain claims.

Assistant: grok
Assistant-Session: 01a04cea-f0d6-7ab3-9ffd-881eb6bea6cb

src/user_engine/pep_stance.py

@@ -0,0 +1,22 @@
+"""Published unreachable-engine stance. Must equal pep-stance.yaml."""
+
+from __future__ import annotations
+
+from datetime import timedelta
+
+UNREACHABLE_STANCE = "fail_closed"
+ALLOW_BINDING = "request"
+ALLOW_LIFETIME = timedelta(seconds=30)
+VERDICT_CACHING = "none"
+
+STANCE = {
+    "z0-experimental": UNREACHABLE_STANCE,
+    "z1-operational": UNREACHABLE_STANCE,
+    "z2-protected": UNREACHABLE_STANCE,
+    "z2-continuity": UNREACHABLE_STANCE,
+    "z3-critical": UNREACHABLE_STANCE,
+    "unknown": UNREACHABLE_STANCE,
+    "not-applicable": UNREACHABLE_STANCE,
+}
+
+DEFAULT_STANCE_SCOPE = "unknown"