Prepare for simplifying `SweepSketch`

`SweepSketch` has two internal code paths. All models have so far been
using the more complicated one. This commit updates them to instead use
the more simple one, in preparation of removing the more complicated
one.

models/cuboid/src/lib.rs

@@ -14,7 +14,7 @@ pub fn model(size: impl Into<Vector<3>>, core: &mut fj::core::Core) -> Solid {
     let [x, y, z] = size.into().components;
 
     let bottom_surface = core.layers.topology.surfaces.xy_plane();
-    let sweep_path = Vector::from([Scalar::ZERO, Scalar::ZERO, z]);
+    let sweep_path = Vector::from([Scalar::ZERO, Scalar::ZERO, -z]);
 
     Sketch::empty(&core.layers.topology)
         .add_regions(

models/holes/src/lib.rs

@@ -18,7 +18,10 @@ pub fn model(radius: impl Into<Scalar>, core: &mut fj::core::Core) -> Solid {
     cuboid.update_shell(
         cuboid.shells().only(),
         |shell, core| {
-            let bottom_face = shell.faces().first();
+            let bottom_face = shell
+                .faces()
+                .nth(5)
+                .expect("Expected shell to have bottom face");
             let offset = size / 2.;
             let depth = size / 2.;
 
@@ -32,11 +35,11 @@ pub fn model(radius: impl Into<Scalar>, core: &mut fj::core::Core) -> Solid {
                 core,
             );
 
-            let bottom_face = shell.faces().first();
-            let top_face = shell
+            let bottom_face = shell
                 .faces()
                 .nth(5)
-                .expect("Expected shell to have top face");
+                .expect("Expected shell to have bottom face");
+            let top_face = shell.faces().first();
 
             [shell.add_through_hole(
                 [

models/spacer/src/lib.rs

@@ -18,7 +18,7 @@ pub fn model(
     core: &mut fj::core::Core,
 ) -> Solid {
     let bottom_surface = core.layers.topology.surfaces.xy_plane();
-    let sweep_path = Vector::from([0., 0., height]);
+    let sweep_path = Vector::from([0., 0., -height]);
 
     Sketch::empty(&core.layers.topology)
         .add_regions(

models/split/src/lib.rs

@@ -22,7 +22,7 @@ pub fn model(size: f64, split_pos: f64, core: &mut fj::core::Core) -> Solid {
             let (shell, [face, _]) = shell.split_face(face, line, core);
 
             [shell
-                .sweep_face_of_shell(face, [0., 0., -size / 2.], core)
+                .sweep_face_of_shell(face, [0., 0., size / 2.], core)
                 .shell]
         },
         core,

models/star/src/lib.rs

@@ -41,7 +41,7 @@ pub fn model(
     }
 
     let bottom_surface = core.layers.topology.surfaces.xy_plane();
-    let sweep_path = Vector::from([0., 0., h]);
+    let sweep_path = Vector::from([0., 0., -h]);
 
     Sketch::empty(&core.layers.topology)
         .add_regions(