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/gauss_bonnet.hpp

@@ -56,6 +56,7 @@ inline int genus(const ConformalMesh& mesh)
 
 // ── Left-hand side  Σ(2π − Θ_v) ─────────────────────────────────────────────
 
+/// Sum `Σ_v (2π − Θ_v)` for a raw vertex → angle property map.
 inline double gauss_bonnet_sum(
     const ConformalMesh&                                      mesh,
     const ConformalMesh::Property_map<Vertex_index, double>&  theta)
@@ -66,15 +67,19 @@ inline double gauss_bonnet_sum(
     return s;
 }
 
+/// `gauss_bonnet_sum` for the Euclidean-functional property bundle.
 inline double gauss_bonnet_sum(const ConformalMesh& m, const EuclideanMaps&   mp)
     { return gauss_bonnet_sum(m, mp.theta_v); }
+/// `gauss_bonnet_sum` for the Spherical-functional property bundle.
 inline double gauss_bonnet_sum(const ConformalMesh& m, const SphericalMaps&   mp)
     { return gauss_bonnet_sum(m, mp.theta_v); }
+/// `gauss_bonnet_sum` for the HyperIdeal-functional property bundle.
 inline double gauss_bonnet_sum(const ConformalMesh& m, const HyperIdealMaps&  mp)
     { return gauss_bonnet_sum(m, mp.theta_v); }
 
 // ── Right-hand side  2π · χ(M) ───────────────────────────────────────────────
 
+/// Right-hand side of Gauss-Bonnet: `2π · χ(M)`.
 inline double gauss_bonnet_rhs(const ConformalMesh& mesh)
 {
     return TWO_PI * static_cast<double>(euler_characteristic(mesh));
@@ -82,14 +87,15 @@ inline double gauss_bonnet_rhs(const ConformalMesh& mesh)
 
 // ── Deficit: lhs − rhs  (0 = Gauss–Bonnet satisfied) ─────────────────────────
 
+/// Gauss-Bonnet deficit `lhs − rhs`; zero iff the identity is satisfied.
 template <typename Maps>
 inline double gauss_bonnet_deficit(const ConformalMesh& mesh, const Maps& maps)
 {
     return gauss_bonnet_sum(mesh, maps) - gauss_bonnet_rhs(mesh);
 }
 
-// ── check_gauss_bonnet — throws std::runtime_error if |deficit| > tol ─────────
-
+/// Throws `std::runtime_error` if `|lhs − 2π·χ| > tol`.
+/// Overload accepting a precomputed `lhs`.
 inline void check_gauss_bonnet(const ConformalMesh& mesh,
                                 double lhs,
                                 double tol = 1e-8)
@@ -108,6 +114,7 @@ inline void check_gauss_bonnet(const ConformalMesh& mesh,
     }
 }
 
+/// Throws `std::runtime_error` if Gauss-Bonnet is violated by more than `tol`.
 template <typename Maps>
 inline void check_gauss_bonnet(const ConformalMesh& mesh,
                                 const Maps& maps,
@@ -123,6 +130,9 @@ inline void check_gauss_bonnet(const ConformalMesh& mesh,
 // Only modifies free vertices (v_idx[v] >= 0 for EuclideanMaps / SphericalMaps;
 // always all vertices for the raw property-map overload).
 
+/// Distribute the Gauss-Bonnet deficit uniformly across all `Θ_v`:
+/// add `δ = (lhs − rhs) / V` to every entry so that the identity holds
+/// exactly afterwards.  Overload for a raw property map.
 inline void enforce_gauss_bonnet(
     ConformalMesh&                                           mesh,
     ConformalMesh::Property_map<Vertex_index, double>&       theta)
@@ -136,6 +146,7 @@ inline void enforce_gauss_bonnet(
         theta[v] += delta;
 }
 
+/// Distribute the Gauss-Bonnet deficit uniformly across `maps.theta_v`.
 template <typename Maps>
 inline void enforce_gauss_bonnet(ConformalMesh& mesh, Maps& maps)
 {