3-step interactive flow:
1. Entity pick (first break point recorded at click location)
2. Optional first point refinement
3. Second break point → CmdResult::BreakEntity applied in cmd_result
Geometry:
- Line: project p1/p2 onto line parameter space → two line fragments
- Arc: project p1/p2 to arc angles → trim CCW from p1 to p2
- Circle: same as Arc, converts Circle → Arc
- LwPolyline: nearest-vertex split → two polyline fragments
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
