refactor: move parallel tessellation fns into scene/convert/

scene/tess.rs held the free parallel-tessellation functions
(tessellate_entity, lod_stub_wire, entity_aabb, …) that the giant-file
split had left at the scene root. They belong with the rest of the
tessellation code, so move the file to scene/convert/tess.rs alongside
tessellate.rs / tess_util.rs and re-point the re-export.

Leaves the scene/ root as a consistent set of `impl Scene` method
groups; the free-function outlier is gone.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/convert/mod.rs

@@ -18,6 +18,7 @@ pub mod acis_to_truck {
 }
 pub mod truck_tess;
 pub mod tessellate;
+pub(crate) mod tess;
 pub mod tess_util;
 pub mod solid3d_tess;
 pub mod spline_tess;