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/scene/model/solid_model.rs

@@ -105,7 +105,7 @@ pub fn edge_wires(solid: &Solid) -> Vec<acadrust::entities::Wire> {
     for shell in solid.boundaries() {
         for edge in shell.edge_iter() {
             if let TruckTessResult::Lines(pts, pts_low) =
-                tessellate_edge(&edge, [0.0; 3])
+                tessellate_edge(&edge)
             {
                 if pts.len() < 2 {
                     continue;
@@ -169,7 +169,7 @@ pub fn boolean(_op: Bool, _a: &Solid, _b: &Solid) -> Option<Solid> {
 /// before world_offset is applied by the caller).
 pub fn mesh_from_solid(solid: &Solid, color: [f32; 4]) -> Option<MeshLodSet> {
     use crate::scene::convert::truck_tess::{tessellate_solid, TruckTessResult};
-    match tessellate_solid(solid, [0.0; 3]) {
+    match tessellate_solid(solid) {
         TruckTessResult::Mesh {
             verts,
             verts_low,