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/01-basics.txt

@@ -0,0 +1,15 @@
+# A cache hit is a method that did not run
+
+cacheManager        : org.springframework.cache.concurrent.ConcurrentMapCacheManager
+repository latency  : 200 ms per lookup
+
+--- first call (miss) ---
+returned            : Book[isbn=978-0134685991, title=Effective Java, year=2018]
+elapsed             : 200 ms
+repository calls    : 1
+
+--- second call (hit) ---
+returned            : Book[isbn=978-0134685991, title=Effective Java, year=2018]
+elapsed             : 0 ms
+repository calls    : 1   <- still 1, the method body never ran
+same object?        : true