Changed my mind. Less class, more clean: “M　✦”.
Colored ring/donut/torus now visible.

A colored band representing the future valid play area is now present, thanks to the fresh & popplin ArenaVisRep. Also, drag-scrolling hacked in for debrugging and debragging.

Also prudent:
• Color presets within SceneView cleaned up; they're now readonly properties, and are instance vars instead of statics.
• ArenaVisRep draws in oranange with with difference blending. I'm leaving in the now-getting-pretty-annoying black/white BG color cycle for now just because the difforange looks pretty awesome on it. I did slow it down to 1/4× tho.
Graphics scaled to real-world (inches) size.

Clean-up and extraction are still needed, but I think it'll do okay for now.