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/mesh_io.hpp

@@ -28,26 +28,22 @@
 
 namespace conformallab {
 
-// ── Read ──────────────────────────────────────────────────────────────────────
-//
-// Reads a polygon mesh from file into `mesh` (clears any existing content).
-// Returns true on success, false on failure.
+/// Read a polygon mesh from `filename` into `mesh` (clears existing content).
+/// Returns `true` on success, `false` on failure.
 inline bool read_mesh(const std::string& filename, ConformalMesh& mesh)
 {
     mesh.clear();
     return CGAL::IO::read_polygon_mesh(filename, mesh);
 }
 
-// ── Write ─────────────────────────────────────────────────────────────────────
-//
-// Writes `mesh` to `filename`.  Returns true on success.
+/// Write `mesh` to `filename`.  Returns `true` on success.
 inline bool write_mesh(const std::string& filename, const ConformalMesh& mesh)
 {
     return CGAL::IO::write_polygon_mesh(filename, mesh);
 }
 
-// ── Convenience: throwing wrappers ────────────────────────────────────────────
-
+/// Throwing wrapper around `read_mesh`: returns the mesh by value
+/// or throws `std::runtime_error` on read failure.
 inline ConformalMesh load_mesh(const std::string& filename)
 {
     ConformalMesh mesh;
@@ -56,6 +52,8 @@ inline ConformalMesh load_mesh(const std::string& filename)
     return mesh;
 }
 
+/// Throwing wrapper around `write_mesh`; throws `std::runtime_error` on
+/// write failure.
 inline void save_mesh(const std::string& filename, const ConformalMesh& mesh)
 {
     if (!write_mesh(filename, mesh))