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/serialization.hpp

@@ -43,7 +43,7 @@ namespace conformallab {
 // JSON
 // ════════════════════════════════════════════════════════════════════════════
 
-// Save solver result (+ optional 2D layout) to a JSON file.
+/// Save the Newton-solver result (+ optional 2-D layout) to a JSON file.
 inline void save_result_json(
     const std::string& path,
     const NewtonResult& res,
@@ -80,8 +80,8 @@ inline void save_result_json(
     ofs << std::setw(2) << j << "\n";
 }
 
-// Load DOF vector from a JSON result file.
-// Returns the DOF vector; fills out res fields if non-null.
+/// Load a DOF vector from a JSON result file written by
+/// `save_result_json`.  If `res` is non-null its fields are filled too.
 inline std::vector<double> load_result_json(
     const std::string& path,
     NewtonResult*       res     = nullptr,
@@ -181,7 +181,7 @@ inline std::vector<double> parse_doubles(const std::string& s)
 
 } // namespace detail_xml
 
-// Save solver result (+ optional layout) to an XML file.
+/// Save the Newton-solver result (+ optional layout) to an XML file.
 inline void save_result_xml(
     const std::string& path,
     const NewtonResult& res,
@@ -229,8 +229,9 @@ inline void save_result_xml(
     ofs << "</ConformalResult>\n";
 }
 
-// Load solver result from an XML file written by save_result_xml.
-// Returns the DOF vector; fills res/geom/layout2d if non-null.
+/// Load a DOF vector from an XML result file written by
+/// `save_result_xml`.  If `res`, `geom`, `layout2d` are non-null they
+/// are filled as well.
 inline std::vector<double> load_result_xml(
     const std::string& path,
     NewtonResult*       res      = nullptr,