Unit radius (gray sphere) is overloaded with meanings

The unit radius (aka the gray sphere as rendered by `/debugcolvol`) has a ton of meanings. The task is to find out what it does and perhaps extract some of them to separate variables (defaulting to radius).

Start by tracing the unit radius and the gray sphere rendered by /debugcolvol through the engine to identify each meaning and where it is used. Document the distinct roles before deciding which should become separate variables, and confirm that existing behavior remains unchanged when the new variables default to radius.

Written by the indexing model from the issue text.