docs: TLS auto-cert and lenient client trust

clients/python/PythonClientDesign.md

@@ -112,6 +112,28 @@ Support:
 - TLS channel with default roots,
 - custom root certificate file.
 
+### Trust posture (trust-on-first-use)
+
+The gateway can serve a self-signed certificate it generates itself (it has no
+PKI). grpc-python exposes no per-channel skip-verify hook, so the client cannot
+"accept any certificate" the way the other clients do. Instead, when the channel
+is not plaintext and neither `ca_file` nor `require_certificate_validation` is
+set, the TLS default is **trust-on-first-use**: the client fetches the server's
+presented certificate once via `ssl.get_server_certificate` (an unverified
+probe), pins it as the channel's only trust root, and — because the generated
+certificate always carries a `localhost` SAN — defaults
+`grpc.ssl_target_name_override` to `localhost` when no `server_name_override` was
+supplied (tolerating dial-by-IP or a hostname mismatch). A failed probe is
+surfaced as a transport error naming the endpoint.
+
+To verify the gateway instead:
+
+- set `ca_file` to verify against a specific CA, or
+- set `require_certificate_validation=True` to verify against the system trust
+  roots.
+
+Both bypass the TOFU path.
+
 ## Streaming
 
 Expose `stream_events` as an async iterator. Canceling the task should cancel