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 <noreply@anthropic.com>

src/scene/mesh_model.rs

@@ -21,12 +21,6 @@ pub struct MeshModel {
     pub selected: bool,
 }
 
-impl MeshModel {
-    #[allow(dead_code)]
-    pub const WHITE: [f32; 4] = [1.00, 1.00, 1.00, 1.0];
-    pub const SELECTED: [f32; 4] = [0.15, 0.55, 1.00, 1.0];
-}
-
 /// Bundle of mesh tessellations at different sampling densities, picked
 /// per frame by the render pipeline based on the projected pixel size of
 /// `world_aabb`. Phase 3.4 LOD ladder: