Add cache and memoization behaviors

[kzu]

Summary

Ship a cache behavior over IMemoryCache or HybridCache, and a memoizer whose entries live on the stunt instance. A process-wide dictionary is the wrong store: a shared cache was dropped from Enterprise Library because entries leaked across objects.

API

• Cache behavior: an IMemoryCache or HybridCache, plus a key selector over the invocation. The default key is the member and the argument values.
• Memoizer: stores results for that stunt only. Dropping the stunt drops the entries.

Behavior

• Skip void members and members with ref or out parameters.
• Take a stampede lock so concurrent callers for one key share one execution.
• Store the unwrapped result of Task<T> or ValueTask<T>. A hit returns a completed awaitable of the same declared kind.
• Failures stay uncached unless the caller opts in.
• Sit inside the retry attempt, so a failed attempt is not stored and a later success is.

Acceptance

• Two calls with the same key invoke the target once.
• Concurrent callers for one key invoke the target once.
• A void method and a method with a ref parameter always reach the target.
• A second stunt does not see the first stunt's memoized values.

Composition

Outer to inner: observation, resilience, transaction, cache, target.