docs: give the twelve experiments that had no architecture diagram one

An audit against the standard the series set — concepts, procedure,
commands, architecture diagram, evidence table, terminal output — found the
three new experiments met it while twelve of the original ones had no
diagram at all: A-0, A-1, A-3, A-4, A-5, A-6, A-8, B-0, B-2, B-7, C-2, D-2.

Each now has one drawn from what that experiment actually found, not filler:
A-0 shows sharing going through PostgreSQL rather than between the caches;
A-3 the gap between the 200 and the WAL flush, with both failed injections;
A-5 the three silent injection failures; A-6 the two places latency is
multiplied; B-0 the repository keyed by principal with no session id; B-2
the primary key that causes the overwrite; D-2 why the rolling update
stopped the accident halfway.

Also corrected the index's stale claim of 11 experiments without a
screenshot — it is 14, and the reason is recorded: those experiments were
measured from terminals, the database and logs, and the observability stack
does not scrape Redis, the BFF or PostgreSQL, so there is no console to
photograph.

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

docs/experiment-index.md

@@ -41,7 +41,14 @@
 | `diagrams/*.svg` | 실험별 구조도 12개 |
 | `evidence/*/*.png` | Grafana · 브라우저 스크린샷 |
 
-**스크린샷이 없는 실험은 11개다** — `A-3 A-7 B-3 B-4 B-5 B-6 C-2 D-1 D-3 D-4 후속`.
+**스크린샷이 없는 실험은 14개다** — `A-3 A-7 A-7a B-3 B-4 B-5 B-6 C-2 D-1 D-3 D-4 D-4a 후속 멀티노드`.
+없는 이유는 **관측 대상에 그 지표가 없기 때문**이지 안 찍어서가 아니다 —
+Prometheus 는 keycloak·kubelet·node-exporter·prometheus 만 긁는다
+([`followup/04-observability-gap.txt`](evidence/followup/04-observability-gap.txt)).
+터미널·DB·로그로 측정한 실험은 브라우저에 띄울 화면 자체가 없다.
+
+**구조도(SVG)는 28개이고, 실험 문서 29개 중 28개가 하나 이상을 싣는다.**
+싣지 않는 하나는 후속 요약 문서이며, 세 실험의 구조도를 표로 참조한다.
 그중 B층은 **Prometheus 가 Redis·BFF·PostgreSQL 을 긁지 않아** 만들 수가 없다 —
 [`followup/04-observability-gap.txt`](evidence/followup/04-observability-gap.txt) 에 측정해 두었다.