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/11-stampede.txt

@@ -0,0 +1,11 @@
+# sync = true is the difference between one slow call and sixteen
+
+16 threads call the same key at the same instant, cold cache.
+The method sleeps 300 ms.
+
+@Cacheable("reports")                     -> 16 invocations
+@Cacheable("syncedReports", sync = true)  -> 1 invocation
+
+Without sync, every thread that arrives during the 300 ms window misses and
+runs the method. That is a cache stampede, and it is worst exactly when the
+cache matters most - right after a restart or an eviction.