Implements the main drawing splitting algorithm that orchestrates splitting
a Drawing into multiple pieces along split lines using Clipper2 polygon
clipping. After clipping, recovers original arcs by matching clipped edges
back to perimeter entities, stitches in feature edges from ISplitFeature
where polygon edges lie on split lines, and normalizes each piece's origin.
Key fix from plan: filters rapid-layer entities before ShapeProfile
construction so cutouts are properly separated from perimeters.
Includes 7 integration tests covering vertical/horizontal splits, three-way
splits, property copying, origin normalization, cutout assignment, and
grid (cross) splits.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
