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-d2-version-upgrade.md

@@ -23,6 +23,15 @@
 
 ---
 
+## 구조
+
+![D-2 — 방향에 따라 갈리는 결과](diagrams/d2-version-upgrade.svg)
+
+> 다이어그램 규약은 [`diagrams/_style.md`](diagrams/_style.md).
+> 실험대 전체 구조는 [`diagrams/lab-topology.svg`](diagrams/lab-topology.svg).
+
+---
+
 ## 0. 결론부터
 
 | 확인 | 결과 |