Add an async-aware Proceed extension for behaviors.

[kzu]

Behaviors see a Task or ValueTask as an ordinary return value. A reusable behavior cannot hard-code the result type, and Native AOT cannot look that result up with reflection.

IMethodInvocation.Proceed calls the rest of the pipeline and hands the behavior a settled ProceedOutcome:

return invocation.Proceed(next, async (outcome, again) =>
{
    if (outcome.Exception is IOException)
        return await again();

    return ProceedOutcome.FromValue((int)outcome.Value! + 1, outcome.Elapsed);
});

AsyncRegistry.Register<T>() installs the closed adapters for Task<T> and ValueTask<T>. The generator emits that call from the stunt static constructor. Non-generic Task and ValueTask adapters ship with the library.

Closes #263.

Tests

• Synchronous values, exceptions, retries, ref outputs, and a callback that yields.
• Faulted and canceled tasks keep their original outcome. A null task fails synchronously.
• Two behaviors compose over one ValueTask<T>.
• A class stunt reports a base exception and can adjust a base Task<int> (Castle/ProceedClassTests.cs).
• Generated stunts contain AsyncRegistry.Register<int>() and AsyncRegistry.Register<string>().