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/io/plot_style.rs

@@ -67,7 +67,6 @@ pub struct PlotStyleTable {
 
 impl PlotStyleTable {
     /// Create an identity CTB table (no overrides for any color).
-    #[allow(dead_code)]
     pub fn identity(name: impl Into<String>) -> Self {
         PlotStyleTable {
             name: name.into(),