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/02-put-evict-clear.txt

@@ -0,0 +1,11 @@
+# @Cacheable, @CachePut and @CacheEvict on the same cache
+
+after findBook          : repository calls = 1
+@CachePut wrote          : Book[isbn=978-0134685991, title=Effective Java (3rd ed.), year=2018]
+next findBook returns    : Book[isbn=978-0134685991, title=Effective Java (3rd ed.), year=2018]
+repository calls         : 1   <- @CachePut refreshed the entry, no reload
+
+after @CacheEvict        : findBook -> Book[isbn=978-0134685991, title=Effective Java, year=2018]
+repository calls         : 2   <- the entry was gone, so the method ran again
+
+after allEntries=true    : repository calls = 5   <- both entries were dropped