Extend GPU analytical circle and arc rendering to block inserts

- Emit transformed analytical curves (circles, arcs, ellipses) in block
  expansion as standalone wires rather than merging them into batched line
  strips.
- Keep render_instance unset on analytical curves in block prototypes so
  they route directly to CircleGpu / EllipseGpu instead of BlockWireGpu.
- Support TangentGeom::Circle in extract_circle_instance and wire partitioning.
- Drop analytical tangents on wires clipped by xclip so clipped geometry
  falls back correctly to line segments.
- Add unit test verifying block circles and arcs extract as analytical GPU
  instances.

src/scene/pick/xclip.rs

@@ -197,6 +197,9 @@ pub fn clip_wires(wires: &mut Vec<WireModel>, poly: &[[f64; 2]]) {
             } else {
                 w.pattern_stations.clear();
             }
+            if !w.tangent_geoms.is_empty() {
+                w.tangent_geoms.clear();
+            }
         }
         if !w.fill_tris.is_empty() {
             let local: Vec<[f32; 3]> = (0..w.fill_tris.len())