feat(ext-cache-core): add the caching contract and a Caffeine backend

Split the way flash-ext-data and flash-ext-view are: cache-core defines
Cache, CacheManager, CacheSpec and CacheStats and talks to nothing;
cache-caffeine implements them in process.

    users = require(CacheManager.class).build("users", spec -> spec
            .maxSize(10_000).ttl(Duration.ofMinutes(10)));

    return users.get(id, repo::findById);

get(key, loader) is the only shape most code needs and the only one that is
hard to get right: the loader runs once per key across concurrent callers
rather than each racing its own. A null result stores nothing, because caching
absence is a decision rather than a default.

build(name, spec) is idempotent per name, so two handlers wanting one cache get
one cache without coordinating who creates it. Disagreeing about the spec
throws rather than resolving to whichever handler initialised first, which is a
bug that only surfaces under load.

recordStats() is opt-in — counting is two atomic increments per lookup, and a
cache nobody measures should not pay for numbers nobody reads. Unmeasured
caches return CacheStats.DISABLED rather than zeroes that look like a cold
cache.

Caffeine rather than a hand-rolled LRU: for genuinely low traffic
ConcurrentHashMap::computeIfAbsent is one line and needs no module at all, and
this exists for when that stops being true. W-TinyLFU admission, striped
counters and amortised eviction are not a weekend's work, and getting them
wrong yields a cache slower than no cache. The adapter is deliberately thin —
every method delegates, adding no wrapper, copy or locking of its own.

Caches are dropped through FlashContext.onClose, so values do not outlive the
app holding them. Invisible with one app per process; immediate under test.

flash-ext-cache-redis is designed but not built, and has docs only — no module,
no pom, no source. An empty module that builds an empty jar is dead weight in
the reactor. The docs record what changes once the cache can fail: get() must
decide whether to fall through to the loader, values need a codec,
invalidateAll needs a key prefix that becomes wire contract, and eviction stats
stop meaning anything. Those are decisions that want a real second replica to
check them against.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pom.xml

@@ -38,6 +38,7 @@
         <maven.versions.plugin.version>2.18.0</maven.versions.plugin.version>
         <jmh.version>1.37</jmh.version>
         <junit.version>5.11.0</junit.version>
+        <caffeine.version>3.1.8</caffeine.version>
         <build.helper.plugin.version>3.6.0</build.helper.plugin.version>
     </properties>
 
@@ -67,6 +68,16 @@
                 <artifactId>flash-testing</artifactId>
                 <version>${project.version}</version>
             </dependency>
+            <dependency>
+                <groupId>dev.relism</groupId>
+                <artifactId>flash-ext-cache-core</artifactId>
+                <version>${project.version}</version>
+            </dependency>
+            <dependency>
+                <groupId>dev.relism</groupId>
+                <artifactId>flash-ext-cache-caffeine</artifactId>
+                <version>${project.version}</version>
+            </dependency>
             <dependency>
                 <groupId>dev.relism</groupId>
                 <artifactId>flash-ext-jackson</artifactId>