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/17-async-cache-mode-missing.txt

@@ -0,0 +1,10 @@
+# The same method on the auto-configured Caffeine manager
+
+cacheManager : org.springframework.cache.caffeine.CaffeineCacheManager
+The application started cleanly. Nothing warned about anything.
+
+buildAsync("q3") ->
+  java.lang.IllegalStateException: No Caffeine AsyncCache available: set CaffeineCacheManager.setAsyncCacheMode(true)
+
+Thrown on the first invocation, in production, at whatever hour that
+endpoint first gets traffic.