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>
