docs(doxygen): 100% public-API coverage (228 → 0 undocumented)

Completes the work begun in the previous commit on this branch.  Every
public symbol under code/include/ now carries a brief Doxygen comment
(0 undocumented per scripts/doxygen-coverage.sh, with the `detail::`
implementation namespaces excluded as before).

Trajectory on this branch:
  start (after Doxyfile fix):  24.0 %  (165 / 437 in the no-detail set
                                       was 105 / 437 when detail counted)
  after PR #17 base commit  :  42.4 %  (165 / 396)
  this commit               : 100.0 %  (396 / 396)

Files touched (all .hpp / .h headers under code/include/):
  * cgal/Conformal_map_traits.h
  * clausen.hpp, conformal_mesh.hpp, constants.hpp (already docd)
  * cp_euclidean_functional.hpp, cut_graph.hpp, discrete_elliptic_utility.hpp
  * euclidean_functional.hpp, euclidean_geometry.hpp, euclidean_hessian.hpp
  * fundamental_domain.hpp, gauss_bonnet.hpp
  * hyper_ideal_{functional,geometry,hessian,utility,visualization_utility}.hpp
  * inversive_distance_functional.hpp, layout.hpp
  * matrix_utility.hpp, mesh_builder.hpp, mesh_io.hpp
  * newton_solver.hpp, p2_utility.hpp, period_matrix.hpp, projective_math.hpp
  * serialization.hpp, spherical_functional.hpp, spherical_geometry.hpp
  * spherical_hessian.hpp, viewer_utils.h

CI:
.gitea/workflows/doxygen-pages.yml now enforces
`scripts/doxygen-coverage.sh --threshold 100`, so any future regression
(a new public function landed without a `///` brief) fails the build
before the Doxygen HTML is published to Codeberg Pages.

Doxygen warnings remain at 0.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

code/include/matrix_utility.hpp

@@ -7,12 +7,10 @@
 
 namespace conformallab {
 
-// Find the 4×4 matrix R that maps source points to target points.
-// Each row of `from` / `to` is a homogeneous 4-vector (one point per row).
-// Post-condition: R * from.row(i).T == to.row(i).T for all i.
-//
-// Implementation: R = to^T * (from^T)^{-1}
-// Corresponds to Java MatrixUtility.makeMappingMatrix().
+/// Find the 4×4 matrix `R` that maps each row of `from` (homogeneous
+/// 4-vector) to the corresponding row of `to`: `R · fromᵀ = toᵀ`.
+/// Computed as `R = toᵀ · (fromᵀ)⁻¹`.  Same as Java
+/// `MatrixUtility.makeMappingMatrix()`.
 inline Eigen::Matrix4d makeMappingMatrix(const Eigen::Matrix4d& from,
                                          const Eigen::Matrix4d& to) {
     return to.transpose() * from.transpose().inverse();