feat(render): double-single vertices for the mesh pipeline

3D solid / surface meshes were the last single-f32 path. truck's mesh
tessellation now returns a paired verts_low residual (TruckTessResult::Mesh,
polygon_to_result), MeshModel/MeshLodSet carry it, the model-space offset
and block-instance transforms reconstruct the f64 position and re-split it
after subtracting world_offset, MeshVertex gains position_low (location 3),
and mesh.wgsl reconstructs the eye-relative position. Native SAT / ACIS /
OBJ mesh paths default to a zero residual (no regression — they were never
sub-f32 precise).

With this every world-space pipeline (wire, hatch, image, face3d, mesh)
is full double-single relative-to-eye; large-coordinate jitter is gone.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/io/obj.rs

@@ -103,6 +103,7 @@ pub fn parse_obj(src: &str, color: [f32; 4]) -> Option<MeshModel> {
     Some(MeshModel {
         name: String::new(),
         verts,
+        verts_low: Vec::new(),
         normals: norms,
         indices,
         color,