STEP export: unique ids, plain curves, right revolution senses; seams on split faces

Vendored truck-stepio (writer only): an intersection curve's second surface
was written at the first one's id (duplicate ids for every boolean edge),
and revolved surfaces lost their orientation (the modeling-surface sense was
always true; inverted or mirrored revolutions came out inside out). Export
now writes intersection edges as C1 cubic B-splines through exact points,
so files carry plain 3D curves and no redundant surfaces.

Vendored truck-meshalgo: hinted parameter searches that run off a B-spline's
domain are rejected or clamped (meshing their grid never finished).

STEP import: straight revolution profiles are bounded in the meridian plane
and stop at the axis; loops on revolutions start away from the axis; face
extents include edge end points. Edges between pieces of one split surface
draw as seams.

kernel::step_validate checks a STEP file (duplicate ids, dangling
references, dense numbering); the oracle replay now exports every rebuilt
part, validates it and re-imports it with no warnings and the same volume
(all 32 rebuildable cases pass).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/topo.rs

@@ -239,6 +239,9 @@ pub fn seam_flags(b: &Body, mesh: &Mesh) -> Vec<bool> {
     let tol = (b.size() * 1e-4).max(1e-6);
     let surfs = classify_faces(mesh, nf, tol);
     let verts = face_vertices(mesh, nf);
+    // Faces cut from one surface (an imported face split where it wraps around a closed
+    // surface) carry the very same surface: the edge between them is a seam.
+    let surf_keys: Vec<String> = solid.face_iter().map(|f| format!("{:?}", f.surface())).collect();
     let mut faces_of: HashMap<String, Vec<usize>> = HashMap::new();
     let mut order: Vec<String> = Vec::new();
     for (fi, f) in solid.face_iter().enumerate() {
@@ -258,7 +261,8 @@ pub fn seam_flags(b: &Body, mesh: &Mesh) -> Vec<bool> {
         .enumerate()
         .map(|(ei, k)| match faces_of.get(k).map(Vec::as_slice) {
             Some([a, b2]) => {
-                !matches!(surfs.get(*a), Some(Surf::Plane { .. })) && same_surface(&surfs, &verts, mesh.edges.get(ei), *a, *b2, tol * 10.0)
+                (!matches!(surfs.get(*a), Some(Surf::Plane { .. })) && same_surface(&surfs, &verts, mesh.edges.get(ei), *a, *b2, tol * 10.0))
+                    || (surf_keys.get(*a).is_some_and(|k| Some(k) == surf_keys.get(*b2)) && !matches!(surfs.get(*a), Some(Surf::Plane { .. })))
             }
             _ => false,
         })