ifcgeom: map IfcSphericalSurface so advanced-brep spheres render (#586)

The IfcSphericalSurface mapping was a dead stub returning nullptr (the
0.5-era implementation was left commented out against a removed kernel
API), so any IfcAdvancedBrep whose face lies on a spherical surface
failed to convert - the Sphere case from the Allplan free-form models
attached to #586. Map it to taxonomy::sphere, mirroring the existing
IfcCylindricalSurface and IfcToroidalSurface mappings.

This activates the existing single-face spherical-solid shortcut in the
OpenCascade kernel (BRepPrimAPI_MakeSphere), which is only reachable
from IfcManifoldSolidBrep closed shells and CSG primitives, so it can
only fire on the genuine full-sphere degenerate-seam pattern.

This change was written with AI assistance.

src/ifcgeom/mapping/IfcSphericalSurface.cpp

@@ -24,16 +24,10 @@ using namespace ifcopenshell::geometry;
 #ifdef SCHEMA_HAS_IfcSphericalSurface
 
 taxonomy::ptr mapping::map_impl(const IfcSchema::IfcSphericalSurface* inst) {
-	return nullptr;
-
-	/*
-	gp_Trsf trsf;
-	IfcGeom::Kernel::convert(inst->Position(), trsf);
-
-	// IfcElementarySurface.Position has unit scale factor
-	face = BRepBuilderAPI_MakeFace(new Geom_SphericalSurface(gp::XOY(), inst->Radius() * length_unit_), getValue(GV_PRECISION)).Face().Moved(trsf);
-	return true;
-	*/
+	auto s = taxonomy::make<taxonomy::sphere>();
+	s->radius = inst->Radius() * length_unit_;
+	s->matrix = taxonomy::cast<taxonomy::matrix4>(map(inst->Position()));
+	return s;
 }
 
 #endif