Fade between Calm and Action background music

For now, I will just have the action music playing constantly.
Once I have a starting screen, I will fade between the calm->action music once the gameplay loop begins.

Relevant files: AudioPlayer.cs

Start with AudioPlayer.cs and inspect how the action music is currently started and kept playing. Check the starting-screen and gameplay-loop entry points mentioned in the issue, then confirm completion when the music fades from calm to action as gameplay begins rather than playing action music constantly.

Written by the indexing model from the issue text.