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/hyper_ideal_visualization_utility.hpp

@@ -32,9 +32,7 @@
 
 namespace conformallab {
 
-// ---------------------------------------------------------------------------
-// Circumcenter of three 2-D points
-// ---------------------------------------------------------------------------
+/// Circumcenter of three 2-D points (`a`, `b`, `c`) in the Euclidean plane.
 inline Eigen::Vector2d circumcenter2d(
     const Eigen::Vector2d& a,
     const Eigen::Vector2d& b,
@@ -54,10 +52,8 @@ inline Eigen::Vector2d circumcenter2d(
     return {ux, uy};
 }
 
-// ---------------------------------------------------------------------------
-// 4×4 Lorentz boost: maps the hyperboloid origin e₄=(0,0,0,1) to `center`.
-// `center` must lie on the hyperboloid: center[3]² - ‖center.head<3>()‖² = 1.
-// ---------------------------------------------------------------------------
+/// 4×4 Lorentz boost: maps the hyperboloid origin `e₄ = (0,0,0,1)` to
+/// `center`.  Precondition: `center` lies on the hyperboloid.
 inline Eigen::Matrix4d hyperboloidTranslation(const Eigen::Vector4d& center)
 {
     Eigen::Vector3d p = center.head<3>();
@@ -73,10 +69,8 @@ inline Eigen::Matrix4d hyperboloidTranslation(const Eigen::Vector4d& center)
     return T;
 }
 
-// ---------------------------------------------------------------------------
-// Project a hyperboloid point to the Poincaré disk (jReality convention:
-// add 1 to the w-coordinate, then dehomogenize the spatial part).
-// ---------------------------------------------------------------------------
+/// Project a hyperboloid point `x` onto the Poincaré disk (jReality
+/// convention: add 1 to the w-coordinate, then dehomogenise spatial part).
 inline Eigen::Vector2d toPoincareDisk(const Eigen::Vector4d& x)
 {
     double w = x(3) + 1.0;
@@ -97,6 +91,10 @@ inline Eigen::Vector2d toPoincareDisk(const Eigen::Vector4d& x)
 //
 // Port of HyperIdealVisualizationPlugin.getEuclideanCircleFromHyperbolic()
 // ---------------------------------------------------------------------------
+/// Convert a hyperbolic circle (`center` on the hyperboloid, hyperbolic
+/// `radius`) to the corresponding Euclidean circle in the Poincaré disk;
+/// returns `{cx, cy, r}`.  Port of `HyperIdealVisualizationPlugin
+/// .getEuclideanCircleFromHyperbolic()`.
 inline std::array<double,3> getEuclideanCircleFromHyperbolic(
     const Eigen::Vector4d& center, double radius)
 {