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/05-key-shapes.txt

@@ -0,0 +1,14 @@
+# What SimpleKeyGenerator actually puts in the map
+
+cache "shapes" after three calls with 0, 1 and 2 arguments:
+
+  cache "shapes":
+    key abc                    [String]   ->  one:abc
+    key SimpleKey [abc, 7]     [SimpleKey]   ->  two:abc:7
+    key SimpleKey []           [SimpleKey]   ->  zero
+
+Zero arguments  -> the SimpleKey.EMPTY constant, printed as []
+One argument    -> that argument itself, unwrapped
+Two or more     -> a SimpleKey holding all of them
+
+The method name and the declaring class appear nowhere in the key.