Add caching: the Spring cache abstraction, keys, eviction timing and the self-invocation trap

A Spring Boot 4.1.1 module whose test suite is the evidence for the article: 19 tests
producing 22 transcripts under docs/output/, plus 12 documentation chapters.

Findings the build pins:
- @EnableCaching has no exposeProxy attribute; the widely-copied
  @EnableCaching(exposeProxy = true) does not compile.
- Two methods sharing a cache name and an argument type share a key space, and one
  silently serves the other's answers.
- beforeInvocation = true is NOT deferred by TransactionAwareCacheManagerProxy on
  7.0.9 - doEvict picks evictIfPresent, which the decorator does not intercept.
- Four of five invalid declarations start a clean context and throw at the first call.
- Caffeine on the classpath silently displaces the simple provider.

caching/scripts/capture-diagnostics.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Starts the application on the simple provider, exercises a few cached methods, and dumps the
+# live contents of every cache through /diag/caches.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+PORT=8080
+PIDFILE=/tmp/caching-demo.pid
+
+mvn -B -q -DskipTests package
+nohup java -jar target/caching-1.0.0.jar \
+  --spring.cache.type=simple --server.port=${PORT} > /tmp/caching-demo.log 2>&1 &
+echo $! > "$PIDFILE"
+
+for _ in $(seq 1 60); do
+  curl -sf "http://localhost:${PORT}/actuator/health" >/dev/null 2>&1 && break
+  sleep 1
+done
+
+{
+  echo "# The live contents of every cache, keys included"
+  echo
+  echo "\$ curl -s localhost:${PORT}/diag/warm"
+  curl -s "http://localhost:${PORT}/diag/warm"
+  echo
+  echo
+  echo "\$ curl -s localhost:${PORT}/diag/caches | jq ."
+  curl -s "http://localhost:${PORT}/diag/caches" | python3 -m json.tool
+} > docs/output/23-diagnostics.txt
+
+kill "$(cat "$PIDFILE")" 2>/dev/null || true
+rm -f "$PIDFILE"
+echo "wrote docs/output/23-diagnostics.txt"