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/docs/output/16-autoconfiguration.txt

@@ -0,0 +1,20 @@
+# What @EnableCaching and Boot's auto-configuration put in the context
+
+CacheManager bean        : org.springframework.cache.caffeine.CaffeineCacheManager
+caches known at startup  : [asyncReports]
+
+bean cacheInterceptor                                     present=true
+bean cacheOperationSource                                 present=true
+bean cacheAdvisor                                         present=false
+bean org.springframework.cache.config.internalCacheAdvisor present=true
+
+CacheInterceptor beans   : [cacheInterceptor]
+KeyGenerator beans       : []
+
+--- where the auto-configuration class lives ---
+FOUND   org.springframework.boot.cache.autoconfigure.CacheAutoConfiguration
+absent  org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration
+
+Boot 4 split spring-boot-autoconfigure into per-technology modules. Caching
+auto-configuration now ships in spring-boot-cache, which the
+spring-boot-starter-cache starter pulls in.