docs: add architecture diagrams for the three follow-up experiments

Each of A-7a, B-7a and D-4a had the concepts, procedure, commands and
terminal evidence but no diagram, which the rest of the series carries.

  a7a-cache-temperature.svg  the three cache states and the SQL each issues
  b7a-orphan-lifecycle.svg   ticket structure, accumulation across two
                             rotations, and why TTL is the only signal
  d4a-hook-timeline.svg      the 38m25s gap against the 1-2s one, on one axis

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

docs/experiment-a7a-volatile-cause.md

@@ -9,6 +9,14 @@ A-7 이 **가설로 남긴 것**을 확정하는 실험이다. 확정해 보니
 
 ---
 
+## 구조
+
+![A-7a — 캐시 온도가 결과를 가른다](diagrams/a7a-cache-temperature.svg)
+
+> 다이어그램 규약은 [`diagrams/_style.md`](diagrams/_style.md).
+
+---
+
 ## 0. 결론부터
 
 | 확인 | 결과 |