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/snap/mod.rs

@@ -162,10 +162,6 @@ impl Snapper {
         self.grid_snap_on = !self.grid_snap_on;
     }
 
-    pub fn set_grid_snap(&mut self, on: bool) {
-        self.grid_snap_on = on;
-    }
-
     pub fn toggle(&mut self, t: SnapType) {
         if !self.enabled.remove(&t) {
             self.enabled.insert(t);