Related issue: #32876

Description

Line3.distanceSqToLine3() writes the closest points into the optional c1 and c2 targets. When both segments are degenerate (start equals end), the early return computes the distance with c1.sub( c2 ), so c1 ends up holding p1 - p2 instead of the closest point on this segment. #32878 fixed the same problem for the general case, but this branch was left unchanged.

The fix keeps c1 and c2 as the two points and returns c1.distanceToSquared( c2 ), like the general case does.

The distanceSqToLine3 unit test now covers two degenerate segments and checks c1 and c2. It fails on dev (c1 is (-1, -1, -1) instead of (1, 1, 1)) and passes with this change.