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/shaders/mesh.wgsl

@@ -30,9 +30,10 @@ struct Uniforms {
 var<uniform> u: Uniforms;
 
 struct VertexIn {
-    @location(0) position: vec3<f32>,
-    @location(1) normal:   vec3<f32>,
-    @location(2) color:    vec4<f32>,
+    @location(0) position:     vec3<f32>,
+    @location(1) normal:       vec3<f32>,
+    @location(2) color:        vec4<f32>,
+    @location(3) position_low: vec3<f32>,
 };
 
 struct VertexOut {
@@ -45,7 +46,8 @@ struct VertexOut {
 @vertex
 fn vs_main(v: VertexIn) -> VertexOut {
     var out: VertexOut;
-    out.clip_pos  = u.view_rot * vec4<f32>((v.position - u.eye_high) - u.eye_low, 1.0);
+    let rel = (v.position - u.eye_high) + (v.position_low - u.eye_low);
+    out.clip_pos  = u.view_rot * vec4<f32>(rel, 1.0);
     out.color     = v.color;
     out.normal    = v.normal;
     out.world_pos = v.position;