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: 20 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.
- The documented cache-provider detection order does not match CacheType's enum
  order in 4.1.1: COUCHBASE before INFINISPAN, and CACHE2K before CAFFEINE.
- 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/docs/output/18-provider-detection.txt

@@ -0,0 +1,33 @@
+# Nothing in application.yml selects a provider. Something still chose one.
+
+spring.cache.type          : (not set)
+resolved CacheManager bean : org.springframework.cache.caffeine.CaffeineCacheManager
+
+Caffeine is on this module's classpath because a later chapter needs TTL and
+size bounds. That single dependency moved every cache in the application off
+the ConcurrentHashMap-backed 'simple' provider.
+
+--- the detection order, read out of the enum rather than the documentation ---
+  1 GENERIC
+  2 JCACHE
+  3 HAZELCAST
+  4 COUCHBASE
+  5 INFINISPAN
+  6 REDIS
+  7 CACHE2K
+  8 CAFFEINE
+  9 SIMPLE
+  10 NONE
+
+CacheConfigurations maps CacheType -> configuration class in an EnumMap, so
+the configurations are imported in this declaration order and the first one
+whose @ConditionalOnClass matches registers the CacheManager. The rest back
+off on @ConditionalOnMissingBean.
+
+Spring Boot's reference documentation lists this order as Generic, JCache,
+Hazelcast, Infinispan, Couchbase, Redis, Caffeine, Cache2k, Simple. On
+4.1.1 the enum disagrees in two places: COUCHBASE comes before INFINISPAN,
+and CACHE2K comes before CAFFEINE. The second one is the one that can bite:
+with both on the classpath you get Cache2k, not Caffeine.
+
+Nothing logs the decision at INFO. Set spring.cache.type explicitly.