kernel: SMLib's export keeps every face of a void's shell

The export takes a closed boundary's faces from the side in the empty region and skipped
every use there that is not `same`, a rule meant only for a sheet (a shell that is its own
mate). It tested the chosen side against the mate instead of the shell against itself, so a
cavity's shell, taken from the void's side, lost its faces whose uses run opposite to their
surfaces: "a face no shell uses" for every cut that leaves a void.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QWhMwdqJYQKQq8RCLSL1LT

crates/kernel/src/native/smlib_import_tests.rs

@@ -80,3 +80,16 @@ fn extruded_circles_go_as_cylinders() {
     let check = r.check(solidcraft_smlib::CheckLevel::All).unwrap();
     assert!(check.is_valid(), "{check}");
 }
+
+/// A cavity cut out of a block: SMLib's result lists the cavity's faces on the void's side with
+/// mixed orientations, and every one of them comes back.
+#[test]
+fn a_cavity_comes_back_from_smlib() {
+    let block = box_solid(v(0., 0., 0.), v(40., 40., 40.)).unwrap();
+    let cavity = box_solid(v(25., 25., 25.), v(30., 30., 30.)).unwrap();
+    let r = super::smlib_body(&block).unwrap().boolean(&super::smlib_body(&cavity).unwrap(), solidcraft_smlib::BooleanOp::Subtract).unwrap().unwrap();
+    let back = super::body_from_smlib(&r).unwrap();
+    assert_eq!((back.solid.boundaries().len(), back.face_count()), (2, 12));
+    let got = super::smlib_body(&back).unwrap().volume(1e-7).unwrap();
+    assert!((got - (64_000.0 - 125.0)).abs() < 1e-6, "{got}");
+}

crates/smlib/src/export.rs

@@ -759,7 +759,7 @@ fn native_ordered(arrays: &Arrays) -> Res<(b::Brep, Report, Order)> {
         for fu in shell_faceuses.get(side).ok_or_else(|| invalid("shell"))?.clone() {
             let (f, same) = faceuse(fu)?;
             // A sheet lists both uses of each face in its one shell: keep the `same` one.
-            if side == mate && !same {
+            if mate == s && !same {
                 continue;
             }
             if !face_uses.get(f).is_some_and(|u| u.iter().any(|(o, _)| *o == mate)) {