Supplement to my previous message: when comparing the project I created with the "official" version at GitHub, I see that my instanced scenes have the Node2D icon in the treeview, where the official version of HUD has what looks like a fountain pen icon ((
) and Player has what seem to be a "resize" icon ((
).
Looking at the actual tscn files, it appears that Player is an Area2D, where in my project it's a Node2D. So the problem is ... that I didn't read the directions correctly when creating my scenes. That would explain it,
So I've now either publicly embarrassed myself, or redeemed myself by figuring out what I did wrong on my own. I'll leave that judgment to you.