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/step_in/geom.rs

@@ -684,7 +684,7 @@ fn revolved(curve: mt::Curve, o: mt::Point3, axis: mt::Vector3) -> mt::Surface {
 /// A surface of revolution whose normal at the profile point `(u, 0)` points along `expected`.
 /// The rotation is reversed rather than the surface inverted: truck's inverted `Processor`
 /// swaps the parameters but not the search hints, which breaks meshing.
-fn revolved_oriented(curve: mt::Curve, o: mt::Point3, axis: mt::Vector3, u: f64, expected: mt::Vector3) -> mt::Surface {
+pub(crate) fn revolved_oriented(curve: mt::Curve, o: mt::Point3, axis: mt::Vector3, u: f64, expected: mt::Vector3) -> mt::Surface {
     let s = revolved(curve.clone(), o, axis);
     if s.normal(u, 0.0).dot(expected) >= 0.0 {
         return s;