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/15-providers-and-ttl.txt

@@ -0,0 +1,17 @@
+# TTL and size bounds are the provider's job, not the abstraction's
+
+cacheManager : org.springframework.cache.caffeine.CaffeineCacheManager
+configured   : expireAfterWrite=400ms, maximumSize=3, recordStats
+
+two calls, same key, immediately -> 1 repository calls
+one more call 600 ms later      -> 2 repository calls   <- the entry expired
+
+--- size bound ---
+five distinct keys written, maximumSize = 3
+estimated size after eviction settles : 3
+stats                                 : hits=1 misses=5 evictions=3
+
+The Spring cache abstraction has no TTL, no size limit and no eviction
+policy of its own - it is an interface over whatever you plug in. On the
+default simple provider, a ConcurrentHashMap, an entry stays until something
+evicts it by hand or the process ends.