Frame brokered HTTP responses once

docs/wasm-runtime-architecture.md

@@ -62,6 +62,15 @@ forced by Wasmtime: an `Engine`/`Store` cannot be safely re-created across
 runtime. So the brokers hold the long-lived connection and units respond to
 events the same way they respond to a page request — through a clean worker.
 
+The broker-to-worker hop explicitly sets `GATEWAY_INTERFACE=CGI/1.1`. After
+the Wasm core decodes request parameters it derives the guest's default status
+syntax from that marker (`Status:` for FastCGI, `HTTP/1.1` for direct HTTP).
+The FastCGI transport clears completed stdout/stderr as soon as their records
+are queued and emits no stream records after `FCGI_END_REQUEST`. Together these
+boundaries prevent a forwarded dynamic response from being parsed as body and
+wrapped in a second HTTP response. The TCP and custom-server tests reject an
+inner status line and require the exact dynamic body.
+
 ---
 
 ## 2. The membrane and the DValue ABI