docs: fill the untested items and record why the B layer has no graphs

The forward upgrade to 26.7.3 was zero downtime across 87 samples, and since databasechangelog stayed at 210 the rollback to 26.7.0 also succeeded, which narrows D-2's conclusion: rolling back fails when the schema moved, not because of the version number. The row count is the check.

Role changes never reach the upstream through request repetition; the session is a snapshot taken at login and only a new session picks up the new claim. Auditing the docs also surfaced that Prometheus scrapes only keycloak, kubelet, node-exporter and itself, so the B-layer experiments have no metrics to screenshot rather than missing screenshots.

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

docs/experiment-b6-key-rotation.md

@@ -12,6 +12,15 @@
 
 ---
 
+## 구조
+
+![B-6 구조 — 겹침 구간과 제거 시점](diagrams/b6-key-rotation.svg)
+
+> 다이어그램 규약은 [`diagrams/_style.md`](diagrams/_style.md).
+> 실험대 전체 구조는 [`diagrams/lab-topology.svg`](diagrams/lab-topology.svg).
+
+---
+
 ## 0. 결론부터
 
 **질문이 두 갈래로 나뉜다.**