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/06-key-collision.txt

@@ -0,0 +1,25 @@
+# The collision the default key generator makes easy
+
+countLetters(String) and countDigits(String) both write into cache "shared".
+
+countLetters("a1b2") -> letters=2   (repository calls: letters=1 digits=0)
+countDigits("a1b2")  -> letters=2   (repository calls: letters=1 digits=0)
+
+countDigits never ran. It found the key "a1b2" already populated and
+returned the answer to a different question.
+
+  cache "shared":
+    key a1b2                   [String]   ->  letters=2
+
+--- no-argument methods collide even harder ---
+currentBanner() -> banner-from-currentBanner
+currentFooter() -> banner-from-currentBanner   <- both key on SimpleKey.EMPTY
+  cache "noargs":
+    key SimpleKey []           [SimpleKey]   ->  banner-from-currentBanner
+
+--- the fix: put the method into the key ---
+countLettersScoped("a1b2") -> letters=2
+countDigitsScoped("a1b2")  -> digits=2
+  cache "scoped":
+    key digits:a1b2            [String]   ->  digits=2
+    key letters:a1b2           [String]   ->  letters=2