chore: drop dead scene/IO helpers + simplify XATTACH command state

* helpers::wcs_to_ucs — UCS inverse transform with zero call sites.
* scene::Scene::compute_and_set_world_offset — never called; the load
  path uses the free function `compute_world_offset` directly.
* tessellate::tessellate_mesh — never called; mesh tessellation goes
  through `solid3d_tess`, not the truck Shell/Solid path.
* truck_tess::tess_to_mesh_model — orphaned with tessellate_mesh's
  removal.
* MeshModel::WHITE / MeshModel::SELECTED — only used by the removed
  tessellate_mesh.
* xattach::Step::FilePath variant — never constructed; XATTACH always
  enters through `with_path` after the file picker. Replaced the Step
  enum + prefilled_path field with flat `path` / `block_name` fields
  and dropped the unreachable text-input flow.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/app/helpers.rs

@@ -46,29 +46,6 @@ pub(super) fn ucs_to_wcs(pt: glam::Vec3, ucs: &Ucs) -> glam::Vec3 {
     o + x * pt.x + y * pt.y + z_ax * pt.z
 }
 
-/// Convert a WCS point back to UCS local coordinates.
-#[allow(dead_code)]
-pub(super) fn wcs_to_ucs(pt: glam::Vec3, ucs: &Ucs) -> glam::Vec3 {
-    let o = glam::Vec3::new(
-        ucs.origin.x as f32,
-        ucs.origin.y as f32,
-        ucs.origin.z as f32,
-    );
-    let x = glam::Vec3::new(
-        ucs.x_axis.x as f32,
-        ucs.x_axis.y as f32,
-        ucs.x_axis.z as f32,
-    );
-    let y = glam::Vec3::new(
-        ucs.y_axis.x as f32,
-        ucs.y_axis.y as f32,
-        ucs.y_axis.z as f32,
-    );
-    let z_ax = ucs_z_axis(ucs);
-    let d = pt - o;
-    glam::Vec3::new(d.dot(x), d.dot(y), d.dot(z_ax))
-}
-
 /// Return the normalised Z axis of a UCS (cross product of X and Y axes).
 pub(super) fn ucs_z_axis(ucs: &Ucs) -> glam::Vec3 {
     let x = glam::Vec3::new(