Validate that half-edges in a cycle don't intersect

[hannobraun]

About

Half-edges in a cycle must not intersect, if the shape that contains the cycle is to be valid. This is currently neither documented nor enforced.

There should be a validation check on the cycle level that makes sure that none of the half-edges with the cycles intersect.

Implementation

The new validation check should go into validate::cycle the existing check there (and the numerous checks in the sibling modules) can be used as inspiration on how to implement it.

Related Issues

There are a few related issues currently open:

• Separate geometry from topology #2116 and Consider uniform intermediate representation for geometry #2118 concern fundamental changes to how geometry is handled. If those were implemented first, this should simplify the implementation of this issue.
• Intersecting half-edges cause panic #2124 is related to a bug triggered by exactly what this issue is supposed to address. Once this issues has been addressed, we can probably close the other one too (at least we'd have to check).
• Consider removing region interiors #2132 would simplify Region, reducing it to a having single cycle. If that were implemented, then implementing this issue would take care of all cycle intersection validation. If not, we'd need a follow-up issue to this one, to check for intersections between different cycles within a region.
• Improve validation infrastructure #2157 is about a refactor of the validation infrastructure. If that were implemented first, that would change where the new validation check would go, and how specifically it would be implemented.

None of those other issues are blockers, but in case work on them is ongoing by the time this issue is picked up, this would require coordination of the work.

[hannobraun]

The validation code will most likely be replaced when the experimental architecture is merged back into the mainline code. This issue may still apply then (the experimental code has no validation code to speak of, so far), but I think it would be a waste of time to fix this before then.

Labeling as "blocked" until the experimental code is merged.