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/04-non-public-and-postconstruct.txt

@@ -0,0 +1,13 @@
+# Two more places the annotation is ignored without a warning
+
+@Cacheable on a protected method, called twice -> 2 repository calls
+No warning is logged. In proxy mode the annotation is only honoured on
+public methods; a protected one is simply never advised.
+
+--- @EnableCaching attributes, as the class file declares them ---
+  AdviceMode mode()
+  int order()
+  boolean proxyTargetClass()
+
+There is no exposeProxy attribute, so @EnableCaching(exposeProxy = true)
+- which a lot of answers recommend - does not compile.