Kernel: split a body by a plane or another face's surface

split_body(body, tool) divides every face where the tool meets it and
closes each side with the piece of the tool surface inside the body.
- Patches have outer loops and holes.
- Bands go round a cylinder or another surface of revolution.
- Each side falls apart into its connected parts.
- A body the tool misses comes back whole.

Faces that are B-spline sheets but really planes or circular cylinders
(extruded faces) split as the whole plane or cylinder: the tool is
extended, and the field is exact and cheap.

Tests (volumes): a box split by a plane (2400/3600); a holed plate split
through its hole into two equal halves; a box split by a cylinder's side
into the core (pi*16*30) and the rest; a plane that misses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/lib.rs

@@ -52,7 +52,7 @@ pub use ops::{BoolOp, boolean, split_by_plane, transform, transform_matrix};
 pub use polybool::planar_boolean;
 pub use polyhedron::{HalfSpace, convex_polyhedron, draft, offset_faces, shell};
 pub use sew::{EdgeSpec, FaceSpec, SurfSpec, sew};
-pub use splitface::{SplitTool, split_faces};
+pub use splitface::{SplitTool, split_body, split_faces};
 pub use step::step_export;
 pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared, step_orientation_errors, step_validate};
 pub use step_out::{ExportBody, ExportProduct, StepHeader, step_export_bodies, step_export_products};