Add initialState option

It starts off in an `undefined` state, which is just the root state from the perspective of a transition diff. It should give you the option to pass an initial state.

No file or test is named. Start by locating the state-machine construction entry point and the code that computes transition diffs, then inspect existing tests if present. Done means callers can provide an initial state and the first transition diff uses it instead of the undefined root state.

Written by the indexing model from the issue text.