docs: C-2 — nothing propagates because nobody implemented the receiving end

Neither client had a backchannel logout URL and the BFF has no oidcLogout configuration, so the three candidate paths all answer 302, which is the authentication redirect rather than a handler. Setting the URL on the identity provider alone changed nothing: with a live session, logging the user out emptied the Keycloak side and left the Redis session untouched.

Reachability is not the blocker here, since a Keycloak pod fetches the app's public URL with a 200, but that is a property of this tailnet split-DNS lab and is the assumption most likely to fail in production, where it fails silently.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/evidence/c2-backchannel-logout/04-reachability.txt

@@ -0,0 +1,15 @@
+=== IdP 세션은 실제로 끊겼는가 ===
+  keycloak-patterns 세션: 0
+
+=== ★ Keycloak 파드가 app1.hyeonworks.com 에 닿는가 ===
+  DNS 해석:
+    Address: 100.83.212.4
+    
+    Non-authoritative answer:
+    
+  HTTPS 도달:
+    HTTP 200  (0 이면 못 닿음)
+
+=== Keycloak 로그 전체에서 backchannel 흔적 ===
+  keycloak-0: 0 줄
+  keycloak-1: 0 줄