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/fundamental_domain.hpp

@@ -43,6 +43,9 @@ namespace conformallab {
 // FundamentalDomain
 // ─────────────────────────────────────────────────────────────────────────────
 
+/// Fundamental polygon of a closed surface obtained by cutting along a
+/// `CutGraph`: corner vertices, paired-edge identifications and holonomy
+/// generators.  For genus-1 the polygon is a parallelogram with 4 corners.
 struct FundamentalDomain {
     /// Polygon corners in order (CCW).  Size = 4 for genus-1.
     std::vector<Eigen::Vector2d> vertices;
@@ -56,6 +59,7 @@ struct FundamentalDomain {
     /// For genus-1: generators[0] = ω_1, generators[1] = ω_2.
     std::vector<Eigen::Vector2d> generators;
 
+    /// `true` iff the polygon has at least 3 vertices.
     bool is_valid() const { return vertices.size() >= 3; }
 };
 
@@ -75,6 +79,8 @@ struct FundamentalDomain {
 //   bottom (v0→v1) ≡ top    (v3→v2)   by ω_2
 //   left   (v3→v0) ≡ right  (v2→v1)   by ω_1   (reversed convention)
 // ─────────────────────────────────────────────────────────────────────────────
+/// Build the parallelogram fundamental domain from genus-1 Euclidean
+/// holonomy data (`hol.translations[0] = ω₁`, `hol.translations[1] = ω₂`).
 inline FundamentalDomain compute_fundamental_domain_genus1(
     const HolonomyData& hol)
 {
@@ -148,6 +154,8 @@ inline FundamentalDomain compute_fundamental_domain_genus1(
 //   this is intentionally deferred and NOT implemented here.
 //   See period_matrix.hpp for the genus-1 case (τ = ω_2/ω_1 ∈ ℍ).
 // ─────────────────────────────────────────────────────────────────────────────
+/// Dispatcher: for genus 1 returns `compute_fundamental_domain_genus1`,
+/// for higher genus returns an empty domain (4g-polygon not yet implemented).
 inline FundamentalDomain compute_fundamental_domain(
     const HolonomyData& hol)
 {
@@ -165,6 +173,8 @@ inline FundamentalDomain compute_fundamental_domain(
 // return a translated copy of the layout shifted by m·ω_1 + n·ω_2.
 // Useful for visualising the tiled universal cover.
 // ─────────────────────────────────────────────────────────────────────────────
+/// Return a translated copy of `layout` shifted by `m·ω₁ + n·ω₂`.
+/// Useful for visualising the tiled universal cover.
 inline Layout2D tiling_copy(const Layout2D& layout,
                              const Eigen::Vector2d& w1,
                              const Eigen::Vector2d& w2,
@@ -183,6 +193,8 @@ inline Layout2D tiling_copy(const Layout2D& layout,
 // Returns a vector of tiling copies for (m, n) with |m| ≤ m_max, |n| ≤ n_max.
 // The result includes the original (m=0, n=0) at index (m_max)(2*n_max+1)+n_max.
 // ─────────────────────────────────────────────────────────────────────────────
+/// Build a tiling neighbourhood: all `(m, n)` with `|m| ≤ m_max`,
+/// `|n| ≤ n_max`.  The original tile `(0, 0)` is included.
 inline std::vector<Layout2D> tiling_neighbourhood(
     const Layout2D&    layout,
     const HolonomyData& hol,