S17/S18: send success and failure routes; Sphinx HTML/PDF

Python router + Go sm-leaf reject missing to, empty ct, plaintext body.
Summary logs never include ciphertext. MODULE.md.

python/secure_messaging/__init__.py

@@ -4,6 +4,7 @@ from .envelope import PassthroughEnvelope  # dest-in-clear, body ciphertext
 from .error_bundle import NetworkErrorBundle  # system-key + sender-only
 from .admin_history import AdminHistory  # DataCube-shaped append-only config log
 from .npe_adapter import NpeRequired  # fail-closed NPE
+from .router import RouteResult, handle_send  # success/failure send policy
 
 __all__ = [
     "SignedConfig",
@@ -12,4 +13,6 @@ __all__ = [
     "NetworkErrorBundle",
     "AdminHistory",
     "NpeRequired",
+    "RouteResult",
+    "handle_send",
 ]