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/viewport.rs

@@ -548,12 +548,10 @@ crate::impl_entity_basics!(Viewport);
 impl crate::entities::traits::FallbackTess for Viewport {
     fn fallback_geometry(
         &self,
-        world_offset: [f64; 3],
     ) -> crate::scene::convert::tess_util::FallbackGeometry {
-        let [ox, oy, oz] = world_offset;
-        let cx = self.center.x - ox;
-        let cy = self.center.y - oy;
-        let cz = self.center.z - oz;
+        let cx = self.center.x;
+        let cy = self.center.y;
+        let cz = self.center.z;
         let hw = self.width / 2.0;
         let hh = self.height / 2.0;
         let pts = vec![