refactor(api): consistent naming for spherical + hyper-ideal helpers (A1–A3)

Standardize the low-level free-function API on <verb>_<geom>_<rest>,
matching the already-consistent setup_<geom>_maps. Old names kept as
[[deprecated]] inline aliases for one release; all internal call sites
migrated.

Renames:
  assign_vertex_dof_indices      -> assign_spherical_vertex_dof_indices
  assign_all_spherical_dof_indices -> assign_spherical_all_dof_indices
  assign_all_dof_indices         -> assign_hyper_ideal_all_dof_indices
  compute_lambda0_from_mesh      -> compute_spherical_lambda0_from_mesh
  gradient_check                 -> gradient_check_hyper_ideal

A4/A5 (public CGAL API) intentionally deferred pending the license/
provenance decision (see CGAL submission audit G0/G1).

Verified: 277/277 CGAL tests pass, no deprecation warnings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

code/tests/cgal/test_phase6.cpp

@@ -434,7 +434,7 @@ static Layout2D make_hyper_ideal_layout_normalised(bool normalise)
 {
     auto mesh = make_triangle();
     auto maps = setup_hyper_ideal_maps(mesh);
-    int n = assign_all_dof_indices(mesh, maps);
+    int n = assign_hyper_ideal_all_dof_indices(mesh, maps);
 
     std::vector<double> xbase(static_cast<std::size_t>(n));
     for (auto v : mesh.vertices()) {