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 <[email protected]>

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;

src/scene/convert/tess.rs

@@ -1,5 +1,5 @@
 // Auto-split from scene/mod.rs. Pure text-move; behaviour unchanged.
-use super::*;
+use super::super::*;
 
 // ── Parallel tessellation free function ──────────────────────────────────────
 //

src/scene/mod.rs

@@ -25,11 +25,11 @@ mod paper;
 mod preview;
 mod project;
 mod selection;
-mod tess;
 
-// Free functions moved into `tess` but still referenced unqualified by this
-// root and by sibling topic modules (each does `use super::*`).
-pub(crate) use tess::{
+// Parallel tessellation free functions live in `convert::tess` (alongside the
+// other tessellation code); re-exported here so this root and sibling topic
+// modules (each does `use super::*`) keep referencing them unqualified.
+pub(crate) use convert::tess::{
     entity_aabb, entity_world_aabb_f64, is_unindexable_entity, tessellate_entity,
     tessellate_entity_dim_text,
 };