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/model/mesh_model.rs

@@ -9,8 +9,11 @@
 pub struct MeshModel {
     /// Unique identifier (entity handle value as decimal string).
     pub name: String,
-    /// World-space vertex positions.
+    /// World-space vertex positions (high half of the double-single pair).
     pub verts: Vec<[f32; 3]>,
+    /// Low residual paired with `verts` so meshes stay precise at UTM scale.
+    /// Empty = all-zero (legacy / interactive meshes near the origin).
+    pub verts_low: Vec<[f32; 3]>,
     /// Per-vertex normals (may be empty if not available).
     pub normals: Vec<[f32; 3]>,
     /// Triangle indices into `verts` (every 3 values = one triangle).

src/scene/model/solid_model.rs

@@ -172,12 +172,14 @@ pub fn mesh_from_solid(solid: &Solid, color: [f32; 4]) -> Option<MeshLodSet> {
     match tessellate_solid(solid, [0.0; 3]) {
         TruckTessResult::Mesh {
             verts,
+            verts_low,
             normals,
             indices,
         } if !indices.is_empty() => {
             let mesh = MeshModel {
                 name: String::new(),
                 verts,
+                verts_low,
                 normals,
                 indices,
                 color,