S14: NPE fail-closed adapter; npe mode never falls back to lab-xor

python/secure_messaging/envelope.py

@@ -75,8 +75,10 @@ def seal(
             raise ValueError("lab_key required for lab-xor")
         key = hashlib.sha256(lab_key).digest()  # 32-byte key
         ct = _xor(key, raw).hex()  # hex ct
-    elif mode == "npe":  # production — sidecar not in this module
-        raise NotImplementedError("npe: attach NPE sidecar; do not HPKE here")
+    elif mode == "npe":  # production — sidecar only
+        from .npe_adapter import seal_npe  # fail-closed import
+
+        ct = seal_npe(to, body)  # never lab-xor here
     else:  # unknown
         raise ValueError("unknown crypto.mode " + mode)
     token = hashlib.sha256(nonce + b"error-token").hexdigest()[:32]  # return-path token stub