Add caching decorator with HybridCache and FusionCache support

[iancooper]

Summary

Provide a caching decorator so query results can be cached declaratively via an attribute like [CacheableQuery].

Context

From V5 discussion #273. Caching is one of the most common cross-cutting concerns for queries and is a natural fit for Darker's decorator pipeline.

Proposal

• Create a [CacheableQuery] attribute with configurable expiry, cache key strategy, etc.
• Create a CacheableQueryDecorator that checks the cache before invoking next, and populates the cache after
• Support Microsoft's HybridCache (new in .NET 9) as the primary caching abstraction
• Consider also supporting FusionCache as a popular alternative
• Cache key generation should be pluggable (default: serialize the query object)
• Support cache invalidation patterns (e.g. via QueryContext.Bag)
• Package as Paramore.Darker.Caching or similar

Design Considerations

• Queries are inherently cacheable (read-only, deterministic for the same input)
• The decorator pattern makes this clean: cache check in Execute, populate on next return
• Cache key must account for all query properties that affect the result