AppContextAsyncProviderLookup

AppContextAsyncProviderLookup is a helper trait used by InAppContext to search for async providers with correct priority:

1. AppContextAsyncProviders in enclosing scope (highest)
2. Local AppContextProvider (sync) converted via fromSyncProvider
3. AppContextAsyncProvider[F, T] defined in T's companion object (lowest)

This works because implicit search looks in the companion of the result type (T), but AppContextAsyncProviderLookup[F, T]'s companion is AppContextAsyncProviderLookup, not T. So givens defined here have priority over T's companion.