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/image_model.rs

@@ -41,7 +41,6 @@ impl ImageModel {
     /// Returns `None` if the image file cannot be opened or decoded.
     pub fn from_raster_image(
         img: &acadrust::entities::RasterImage,
-        world_offset: [f64; 3],
     ) -> Option<Self> {
         let w = img.size.x;
         let h = img.size.y;
@@ -50,9 +49,9 @@ impl ImageModel {
         // Corners come from a large insertion point plus small u/v spans.
         // Split each into double-single (high, low) f32 so the GPU keeps
         // sub-unit precision at UTM scale and after a cross-drawing paste.
-        let oxv = img.insertion_point.x - world_offset[0];
-        let oyv = img.insertion_point.y - world_offset[1];
-        let ozv = img.insertion_point.z - world_offset[2];
+        let oxv = img.insertion_point.x;
+        let oyv = img.insertion_point.y;
+        let ozv = img.insertion_point.z;
         let ux = (img.u_vector.x * w) as f32;
         let uy = (img.u_vector.y * w) as f32;
         let uz = (img.u_vector.z * w) as f32;