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_builder.hpp

@@ -22,7 +22,7 @@ namespace conformallab {
 //   |    \
 //   v0 ─ v1
 //
-// Returns a mesh with 1 face, 3 vertices, 3 edges.
+/// Build a single right-angle triangle in the xy-plane (1 face, 3 vertices, 3 edges).
 // The triangle lies in the xy-plane with a right angle at v0.
 inline ConformalMesh make_triangle(
     double x0=0, double y0=0,
@@ -39,7 +39,7 @@ inline ConformalMesh make_triangle(
 
 // ── Regular tetrahedron ──────────────────────────────────────────────────────
 //
-// 4 vertices, 4 faces, 6 edges.
+/// Build a regular tetrahedron (4 vertices, 4 faces, 6 edges; sphere topology).
 // Euler characteristic: V - E + F = 4 - 6 + 4 = 2  (sphere topology).
 // Used to test closed-surface traversal.
 inline ConformalMesh make_tetrahedron()
@@ -67,8 +67,8 @@ inline ConformalMesh make_tetrahedron()
 //   |  \ |
 //   v0 ─ v1
 //
-// 4 vertices, 2 faces, 5 edges (1 interior edge v1–v2 shared by both faces).
-// Useful for testing edge-interior vs edge-boundary distinction.
+/// Build a two-triangle strip (4 vertices, 2 faces, 5 edges; 1 interior edge).
+/// Useful for testing interior- vs boundary-edge distinction.
 inline ConformalMesh make_quad_strip()
 {
     ConformalMesh mesh;
@@ -85,8 +85,8 @@ inline ConformalMesh make_quad_strip()
 
 // ── Regular flat polygon fan ─────────────────────────────────────────────────
 //
-// n triangles sharing a central vertex; forms a disk topology (boundary).
-// Used to verify valence-n vertex traversal.
+/// Build a regular flat polygon fan: `n` triangles sharing a central
+/// vertex, with rim vertices on the unit circle (disk topology).
 inline ConformalMesh make_fan(int n)
 {
     CGAL_precondition(n >= 3);
@@ -109,10 +109,9 @@ inline ConformalMesh make_fan(int n)
 
 // ── Spherical tetrahedron (vertices on the unit sphere) ───────────────────────
 //
-// The four vertices of a regular tetrahedron projected onto the unit sphere.
-// Starting from (±1,±1,±1), dividing by √3 gives unit-length positions.
-// All edge lengths equal arccos(−1/3) ≈ 1.9106 radians.
-// Used for SphericalFunctional tests (all four faces are valid spherical triangles).
+/// Build a regular tetrahedron with vertices on the unit sphere.
+/// All edge lengths equal `arccos(−1/3) ≈ 1.9106 rad`; used by the
+/// SphericalFunctional tests.
 inline ConformalMesh make_spherical_tetrahedron()
 {
     ConformalMesh mesh;
@@ -133,10 +132,9 @@ inline ConformalMesh make_spherical_tetrahedron()
 
 // ── Octahedron face triangle (vertices on the unit sphere) ────────────────────
 //
-// One face of a regular octahedron: the triangle (1,0,0)→(0,1,0)→(0,0,1).
-// All edge lengths equal arccos(0) = π/2.
-// The corner angles are all π/2 (right-angled spherical triangle).
-// base log-length: λ° = 2·log(sin(π/4)) = 2·log(1/√2) = −log(2) ≈ −0.6931.
+/// Build one face of a regular octahedron `(1,0,0)→(0,1,0)→(0,0,1)`:
+/// a right-angled spherical triangle with edge length `π/2` and base
+/// log-length `λ° = −log 2 ≈ −0.6931`.
 inline ConformalMesh make_octahedron_face()
 {
     ConformalMesh mesh;