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/leader.rs

@@ -567,7 +567,6 @@ pub trait LeaderTess {
         selected: bool,
         entity_color: [f32; 4],
         line_weight_px: f32,
-        world_offset: [f64; 3],
         anno_scale: f32,
     ) -> crate::scene::model::wire_model::WireModel;
 }
@@ -580,7 +579,6 @@ impl LeaderTess for Leader {
         selected: bool,
         entity_color: [f32; 4],
         line_weight_px: f32,
-        world_offset: [f64; 3],
         anno_scale: f32,
     ) -> crate::scene::model::wire_model::WireModel {
         use crate::scene::convert::tessellate::{append_arrow, arrow_from_block, ArrowKind, DimGeom};
@@ -591,7 +589,7 @@ impl LeaderTess for Leader {
             entity_color
         };
         let name = handle.value().to_string();
-        let [ox, oy, oz] = world_offset;
+        let [ox, oy, oz] = [0.0_f64; 3];
         let p3 = |v: &acadrust::types::Vector3| -> [f32; 3] {
             [(v.x - ox) as f32, (v.y - oy) as f32, (v.z - oz) as f32]
         };