isImplementation does not check for input enabledness

The `TransitionSystem.isImplementation()` method does not check whether the transition system is input enabled, which is a theoretical requirement.

Start at the TransitionSystem.isImplementation() entry point and determine how input enabledness is represented in the model-checking engine. Verify the theoretical requirement in the surrounding implementation and tests, then ensure implementation checks account for it and add or update coverage. Done means isImplementation() rejects systems that are not input enabled.

Written by the indexing model from the issue text.