docs: record the proxy's failover behaviour and how to verify it

serverAppConfigs-docs/LLM/frank-medium-radiance-mxfp4.md

@@ -226,6 +226,25 @@ The same call works against either replica directly (`:7771`, `:7772`) — when
 one of them is down the other still serves, which is why the proxy reports the
 individual replica states instead of a single boolean.
 
+A replica that dies mid-flight does not cost a client a request: a failed
+connection demotes that replica immediately and the proxy retries on the live
+one (vLLM's endpoints are stateless, so re-sending a request that never reached
+an engine is safe) and logs the failover. Verify with card 1 stopped:
+
+```
+systemctl --user stop frank-medium-a.service
+for i in 1 2 3 4 5 6; do
+  curl -s -m 30 -o /dev/null -w "%{http_code} " -H 'Content-Type: application/json' \
+    -d '{"model":"frank-medium","messages":[{"role":"user","content":"probe"}],"max_tokens":8}' \
+    http://127.0.0.1:7770/v1/chat/completions
+done; echo
+podman logs --since 60s frank-medium-proxy | grep -i failover
+systemctl --user start frank-medium-a.service
+```
+
+Six 200s and one `proxy: failover — … unreachable` line is the pass condition.
+Restarting a stopped replica takes about 150 s (warm compile cache).
+
 ## Benchmarks
 
 `vllm bench serve` against the proxy (both cards serving), random dataset,