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/euclidean_geometry.hpp

@@ -29,19 +29,17 @@
 
 namespace conformallab {
 
+/// Interior corner angles of a Euclidean triangle.
 struct EuclideanFaceAngles {
-    double alpha1;   ///< corner angle at v1 (opposite l23)
-    double alpha2;   ///< corner angle at v2 (opposite l31)
-    double alpha3;   ///< corner angle at v3 (opposite l12)
-    bool   valid;
+    double alpha1;   ///< Corner angle at v₁ (opposite l₂₃).
+    double alpha2;   ///< Corner angle at v₂ (opposite l₃₁).
+    double alpha3;   ///< Corner angle at v₃ (opposite l₁₂).
+    bool   valid;    ///< `false` when the triangle is degenerate.
 };
 
-// ── From side lengths ─────────────────────────────────────────────────────────
-//
-// Given three Euclidean side lengths l12, l23, l31 > 0 satisfying the triangle
-// inequality, compute the corner angles.
-//
-// Returns valid=false if the triangle inequality is violated (any t-value ≤ 0).
+/// Compute the corner angles of a Euclidean triangle from its three
+/// side lengths.  Returns `valid = false` when the triangle inequality
+/// is violated.
 inline EuclideanFaceAngles euclidean_angles_from_lengths(
     double l12, double l23, double l31)
 {
@@ -70,14 +68,9 @@ inline EuclideanFaceAngles euclidean_angles_from_lengths(
     };
 }
 
-// ── From effective log-lengths Λ̃ ─────────────────────────────────────────────
-//
-// Converts to side lengths l_ij = exp(Λ̃_ij / 2), applying the centering
-// trick for numerical safety, then delegates to euclidean_angles_from_lengths.
-//
-// The centering constant μ = (Λ̃12 + Λ̃23 + Λ̃31) / 6 ensures
-//   l12 · l23 · l31 = 1  (geometric mean = 1)
-// which keeps all l values near 1 and prevents float overflow for large |Λ̃|.
+/// Compute the corner angles of a Euclidean triangle from its three
+/// effective log-lengths `Λ̃ᵢⱼ`.  Internally centres lengths so that
+/// `l₁₂·l₂₃·l₃₁ = 1` to avoid float overflow for large `|Λ̃|`.
 inline EuclideanFaceAngles euclidean_angles(
     double lam12, double lam23, double lam31)
 {