refactor: remove dead world_offset plumbing (RTE migration is complete)

Now that geometry reaches the GPU/CPU as absolute coordinates via the
double-single relative-to-eye path, world_offset is always [0,0,0]. Strip
the parameter that was threaded through the whole tessellation / block-
expansion / fallback / camera-decode chain and the ExpandCtx field that
carried it — ~340 references across 20 files, all subtracting zero.

- truck_tess: to_local / to_local_low / tessellate_* drop the offset arg;
  to_local is now a pure double-single split.
- tessellate: offset_to_ds → points_to_ds (pure DS split); tessellate(),
  tessellate_entity(), fallback_geometry(), solid_wire_fallback(),
  entity_aabb(), expand_insert(), expand_block_meshes(), the dimension
  helpers (vec3_local, dimension_snap_pts, …), text_support, leader,
  multileader, image_model, xclip and camera_from_view all lose the param.
- block_cache: ExpandCtx loses its world_offset field.
- Remove the now-dead offset_snap_pts and the unused set_grid_snap.

Behaviour is unchanged (the offset was zero everywhere); the compiler
verifies every call site.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/pick/xclip.rs

@@ -65,9 +65,8 @@ fn dict_entry(doc: &CadDocument, dict: Handle, key: &str) -> Option<Handle> {
 pub fn world_clip_polygon(
     sf: &SpatialFilter,
     ins: &Insert,
-    world_offset: [f64; 3],
 ) -> Vec<[f32; 2]> {
-    world_clip_polygon_f64(sf, ins, world_offset)
+    world_clip_polygon_f64(sf, ins)
         .into_iter()
         .map(|[x, y]| [x as f32, y as f32])
         .collect()
@@ -80,11 +79,10 @@ pub fn world_clip_polygon(
 pub fn world_clip_polygon_f64(
     sf: &SpatialFilter,
     ins: &Insert,
-    world_offset: [f64; 3],
 ) -> Vec<[f64; 2]> {
     let xform = ins.get_transform();
     let inv_block = &sf.inverse_block_transform;
-    let [ox, oy, _] = world_offset;
+    let [ox, oy, _] = [0.0_f64; 3];
     let local: Vec<[f64; 2]> = if sf.boundary_points.len() == 2 {
         let a = sf.boundary_points[0];
         let b = sf.boundary_points[1];