Add a lifecycle contract: adapter dispose() and CacheManager shutdown

[camcima]

Deferred from the architecture review (architectural observation #1) — see ARCHITECTURE_REVIEW.md.

Problem

There is no orderly teardown path through the library's own abstractions:

• CacheAdapter has no dispose()/close() in its contract.
• MemoryAdapter.close() exists (packages/core/src/memory-adapter.ts:34) but only because checkPeriodMs starts a node-cache timer — it is an ad-hoc method, not part of any interface, so nothing generic can call it.
• CacheManager has no shutdown at all. A caller holding a manager has no way to release its layers without reaching into getLayers() and type-testing each one for a method that may not exist.
• ZigguratModule registers no onModuleDestroy/onApplicationShutdown, so a NestJS app's graceful shutdown never reaches the cache.

The practical symptom: a MemoryAdapter with checkPeriodMs set keeps an interval alive, which can hold a Node process open past where it should have exited — in tests, in CLI tools, in serverless teardown. The docs currently tell users to call close() themselves and know which adapters have it.

Sketch

• Optional dispose?(): Promise<void> on the CacheAdapter interface — optional keeps every existing custom adapter valid, the same approach ttlPolicy took in fix: resolve architecture review findings (H1-H3, M1-M6, L1-L6) #56.
• BaseCacheAdapter provides a no-op default so extending adapters get it free.
• MemoryAdapter.dispose() stops the timer (keep close() as a deprecated alias, or fold it in).
• CacheManager.close() fans out to every layer that declares one, allSettled so a failing layer doesn't strand the others.
• ZigguratModule implements OnApplicationShutdown and calls it.

Connection lifecycle stays the caller's job — the Redis/Memcached/SQLite adapters take a client you constructed and should not close it out from under you. Worth stating that explicitly in the docs so the boundary is unambiguous.

Done when

A NestJS app shutting down, or a script calling await cache.close(), releases everything the library started, without the caller knowing which adapter types are in the stack.