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/scene/pipeline/mesh_gpu.rs

@@ -19,30 +19,37 @@ pub struct MeshVertex {
     pub position: [f32; 3],
     pub normal: [f32; 3],
     pub color: [f32; 4],
+    pub position_low: [f32; 3],
 }
 
 impl MeshVertex {
     pub fn layout<'a>() -> wgpu::VertexBufferLayout<'a> {
+        const ATTRS: &[wgpu::VertexAttribute] = &[
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(MeshVertex, position) as u64,
+                shader_location: 0,
+                format: wgpu::VertexFormat::Float32x3,
+            },
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(MeshVertex, normal) as u64,
+                shader_location: 1,
+                format: wgpu::VertexFormat::Float32x3,
+            },
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(MeshVertex, color) as u64,
+                shader_location: 2,
+                format: wgpu::VertexFormat::Float32x4,
+            },
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(MeshVertex, position_low) as u64,
+                shader_location: 3,
+                format: wgpu::VertexFormat::Float32x3,
+            },
+        ];
         wgpu::VertexBufferLayout {
             array_stride: std::mem::size_of::<MeshVertex>() as u64,
             step_mode: wgpu::VertexStepMode::Vertex,
-            attributes: &[
-                wgpu::VertexAttribute {
-                    offset: 0,
-                    shader_location: 0,
-                    format: wgpu::VertexFormat::Float32x3,
-                },
-                wgpu::VertexAttribute {
-                    offset: 12,
-                    shader_location: 1,
-                    format: wgpu::VertexFormat::Float32x3,
-                },
-                wgpu::VertexAttribute {
-                    offset: 24,
-                    shader_location: 2,
-                    format: wgpu::VertexFormat::Float32x4,
-                },
-            ],
+            attributes: ATTRS,
         }
     }
 }
@@ -133,6 +140,7 @@ impl MeshGpu {
                     position: pos,
                     normal,
                     color,
+                    position_low: mesh.verts_low.get(i).copied().unwrap_or([0.0; 3]),
                 }
             })
             .collect();