feat(render): GPU analytical rendering for circles, arcs, and ellipses

- Render planar circles, circular arcs, ellipses, and elliptical arcs analytically via GPU-instanced screen-space quads and WGSL shaders.
- Implement exact arc sweep clamping with round end caps matching the CAD wire shader.
- Implement robust Newton-Raphson closest-point solver for ellipses with isotropic screen-space derivative anti-aliasing.
- Integrate into wire pipeline and PersistentWireArena upload paths.
- Remove unused mesh_volume_centroid function in doc_api.

src/scene/model/wire_model.rs

@@ -47,6 +47,15 @@ pub enum TangentGeom {
         start_angle: f64,
         end_angle: f64,
     },
+    /// Ellipse or elliptical arc in an arbitrary world-space plane.
+    PlanarEllipse {
+        center: [f64; 3],
+        major_axis: [f64; 3],
+        normal: [f64; 3],
+        minor_axis_ratio: f64,
+        start_param: f64,
+        end_param: f64,
+    },
 }
 
 /// Viewport-relative display transform for a point marker.