Add ToLine() to SplitLine and create SplitLineIntersect static class with
FindIntersection, CrossesSplitLine, and SideOf methods for testing entity
intersections against split lines. These helpers support the upcoming
Clipper2-free DrawingSplitter rewrite.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
