fix: world_offset precision fix for large-coordinate DXF files (e.g. Turkish UTM)

Subtracts the DXF header extents centroid (world_offset) from all coordinates
in f64 BEFORE casting to f32. This eliminates the ~0.5 m ULP error that occurs
at 4,000,000 m UTM coordinates and caused geometry to disappear on zoom-out.

- Scene: adds world_offset field + compute_and_set_world_offset() method
- tessellate.rs: threads world_offset through all f64→f32 conversions
  (legacy_geometry, dimension_geometry, solid_wire_fallback, multileader)
- truck_tess.rs: to_local() helper already in place from prior session
- mod.rs: tessellate_entity() and entity_aabb() apply world_offset
- update.rs: compute_and_set_world_offset() called after doc load
- cmd_result.rs / solid3d_cmds.rs: pass correct offset to truck tessellators

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/insert/solid3d_cmds.rs

@@ -25,7 +25,7 @@ use crate::scene::truck_tess;
 // ── Tessellation helper ────────────────────────────────────────────────────
 
 fn solid_to_mesh(solid: &truck_modeling::Solid, color: [f32; 4], name: &str) -> Option<MeshModel> {
-    match truck_tess::tessellate_solid(solid) {
+    match truck_tess::tessellate_solid(solid, [0.0; 3]) {
         truck_tess::TruckTessResult::Mesh { verts, normals, indices } => Some(MeshModel {
             name: name.to_string(),
             verts, normals, indices,
@@ -37,7 +37,7 @@ fn solid_to_mesh(solid: &truck_modeling::Solid, color: [f32; 4], name: &str) ->
 }
 
 fn shell_to_mesh(shell: &truck_modeling::Shell, color: [f32; 4], name: &str) -> Option<MeshModel> {
-    match truck_tess::tessellate_shell(shell) {
+    match truck_tess::tessellate_shell(shell, [0.0; 3]) {
         truck_tess::TruckTessResult::Mesh { verts, normals, indices } => Some(MeshModel {
             name: name.to_string(),
             verts, normals, indices,