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 <noreply@anthropic.com>

src/entities/hatch.rs

@@ -344,14 +344,13 @@ impl Grippable for Hatch {
 }
 
 impl FallbackTess for Hatch {
-    fn fallback_geometry(&self, world_offset: [f64; 3]) -> FallbackGeometry {
-        let [ox, oy, oz] = world_offset;
+    fn fallback_geometry(&self) -> FallbackGeometry {
         let normal = (self.normal.x, self.normal.y, self.normal.z);
-        // Convert a 2D OCS hatch boundary point to WCS, then subtract world_offset.
+        // Convert a 2D OCS hatch boundary point to absolute WCS.
         let to_wcs = |x: f64, y: f64| -> [f64; 3] {
             let (wx, wy, wz) =
                 crate::scene::view::transform::ocs_point_to_wcs((x, y, self.elevation), normal);
-            [wx - ox, wy - oy, wz - oz]
+            [wx, wy, wz]
         };
         // Snap point at a world (f64) location, cast to the f32 snap buffer.
         let snap_at = |w: [f64; 3]| Vec3::new(w[0] as f32, w[1] as f32, w[2] as f32);