refactor, split project structure, reuse explicit imports, fix diameter naming to match actual implementation, nullsafety

[slimlime]

draft pull request

big diversion

have something to track from large refactor way back

move towards split file project structure conventions fix radius is actually diameter bug a quick nullsafety move from before the nullsafety migration

explicit imports

reuse base functionality instead of copying in-line function

still some strange holdovers from legacy that was uncertain of original intent (optional with heaps of constructor body asserts and checks for different state combinations)

[rocboronat]

+1 to that radius -> diameter change :)