docs(keycloak-session-store): import the session-storage lab as a new project

The keycloak project ended with four open questions that design could not
settle. A two-VM lab was built to answer them by measurement, and this is
that material: 26 experiments, 125 raw command outputs, 22 browser captures.

Follows the import procedure in README.md.

  source/     the originating repository verbatim — 78 documents, 28 SVGs,
              8 manifests, plus .source-revision recording the commit
  final/      the SSOT
    document.md   729 lines written from the 29 experiment documents, not
                  concatenated: what was predicted, what was measured, and
                  where the measurement itself was wrong
    evidence/raw    125 outputs, flattened to <experiment>__<file> because
                    the originals collided (01-baseline.txt appeared three
                    times) and the audit only globs the top level
    evidence/meta   one per raw file; command and exitCode are null and the
                    README says why rather than inventing them
    evidence/browser  22 captures
    assets/       three diagrams through techviz
    .techviz/     their VizSpecs

A separate project rather than an addition to keycloak: the B-layer answers
that project's four questions, but the A, C and D layers are about cluster
failure, SSO and operations, and one document.md should hold one subject.
The four question records there can point here through 관계.

Recorded rather than papered over: only three of the 28 diagrams were
remade. The repository forbids hand-drawn SVG and forbids titles inside the
canvas; all 28 originals carry both, so converting them is redrawing, not
reformatting. They stay in source/ and the gap is written into the document.

verify-pipeline.py passes. audit-records.py reports no issues.

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

docs/clean-architecture-backend-template/final/evidence/raw/a05-f004-retrydecision-reason.txt

@@ -0,0 +1,40 @@
+# 사유 필드와 그 문서
+/**
+ * The outcome of classifying one failed attempt (design §9.4).
+ *
+ * <p>The {@code reason} is a bounded diagnostic string chosen by the policy, never a provider
+ * message, so it is safe to log and to use as a low-cardinality metric tag.
+ */
+public record RetryDecision(RetryDisposition disposition, Duration delay, String reason) {
+# 컴팩트 생성자가 검사하는 것 전부
+  public RetryDecision {
+    Objects.requireNonNull(disposition, "disposition");
+    Objects.requireNonNull(delay, "delay");
+    Objects.requireNonNull(reason, "reason");
+    if (delay.isNegative()) {
+      throw new IllegalArgumentException("retry delay must not be negative");
+    }
+    if (disposition != RetryDisposition.RETRY_FULL_TRANSACTION && !delay.isZero()) {
+      throw new IllegalArgumentException("only a full transaction retry may carry a delay");
+    }
+    if (reason.isBlank()) {
+      throw new IllegalArgumentException("retry decision requires a reason");
+    }
+  }
+# 공개 팩토리 넷. 사유를 인자로 받는 것은 셋이다.
+30:  public static RetryDecision retry(Duration delay) {
+36:  public static RetryDecision retry(Duration delay, String reason) {
+41:  public static RetryDecision reconcile(String reason) {
+46:  public static RetryDecision fail(String reason) {
+# 이 파일에서 길이·어휘·형식을 검사하는 코드: 0
+
+# 이 타입을 소비하는 main 파일 전부
+FullTransactionRetryCoordinator.java
+DefaultJpaRetryPolicy.java
+JpaRetryObservation.java
+#   그중 reason() 을 읽는 줄: 0
+
+# 그런데 결정 객체 자체는 메트릭을 내는 곳까지 간다
+106:        listener.onAttemptFailed(operation, attempt, failure, decision);
+41:    LowCardinality.requireRegistered(persistenceUnit);
+58:        .tag("retry.disposition", decision.disposition().name())