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/23-diagnostics.txt

@@ -0,0 +1,34 @@
+# The live contents of every cache, keys included
+
+$ curl -s localhost:8080/diag/warm
+warmed: books, shapes, nulls
+
+$ curl -s localhost:8080/diag/caches | jq .
+{
+    "cacheManager": "org.springframework.cache.concurrent.ConcurrentMapCacheManager",
+    "caches": {
+        "nulls": {
+            "implementation": "org.springframework.cache.concurrent.ConcurrentMapCache",
+            "nativeStore": "java.util.concurrent.ConcurrentHashMap",
+            "entries": {
+                "xyz   [String]": "null   [NullValue]"
+            }
+        },
+        "books": {
+            "implementation": "org.springframework.cache.concurrent.ConcurrentMapCache",
+            "nativeStore": "java.util.concurrent.ConcurrentHashMap",
+            "entries": {
+                "978-0134685991   [String]": "Book[isbn=978-0134685991, title=Effective Java, year=2018]   [Book]"
+            }
+        },
+        "shapes": {
+            "implementation": "org.springframework.cache.concurrent.ConcurrentMapCache",
+            "nativeStore": "java.util.concurrent.ConcurrentHashMap",
+            "entries": {
+                "SimpleKey []   [SimpleKey]": "zero   [String]",
+                "SimpleKey [abc, 7]   [SimpleKey]": "two:abc:7   [String]",
+                "abc   [String]": "one:abc   [String]"
+            }
+        }
+    }
+}