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/p2_utility.hpp

@@ -13,9 +13,9 @@ namespace conformallab {
 
 // ── Point / line duality ──────────────────────────────────────────────────────
 
-// Intersection of two lines l1, l2  (or  line through two points p1, p2)
-// via the cross product.  Works for any P2 element.
-// Corresponds to Java P2.pointFromLines / P2.lineFromPoints.
+/// Cross-product point–line duality in P²: returns the intersection
+/// of two lines (or the line through two points).  Same as Java
+/// `P2.pointFromLines` / `P2.lineFromPoints`.
 inline Eigen::Vector3d pointFromLines(const Eigen::Vector3d& l1,
                                       const Eigen::Vector3d& l2) {
     return l1.cross(l2);
@@ -23,11 +23,9 @@ inline Eigen::Vector3d pointFromLines(const Eigen::Vector3d& l1,
 
 // ── Euclidean perpendicular bisector ─────────────────────────────────────────
 
-// Returns the homogeneous line coordinates (a, b, c) of the perpendicular
-// bisector of the segment [p, q] in the Euclidean plane.
-// Coordinates: ax + by + c = 0  (after dehomogenizing p and q).
-//
-// Corresponds to Java P2.perpendicularBisector(p, q, Pn.EUCLIDEAN).
+/// Homogeneous line coordinates `(a, b, c)` of the perpendicular
+/// bisector of `[p, q]` in the Euclidean plane (`ax + by + c = 0`).
+/// Same as Java `P2.perpendicularBisector(p, q, Pn.EUCLIDEAN)`.
 inline Eigen::Vector3d perpendicularBisectorEuclidean(const Eigen::Vector3d& p_h,
                                                       const Eigen::Vector3d& q_h) {
     // Dehomogenize
@@ -46,8 +44,7 @@ inline Eigen::Vector3d perpendicularBisectorEuclidean(const Eigen::Vector3d& p_h
     return {d(0), d(1), c};
 }
 
-// ── Euclidean distance between two P2 homogeneous points ─────────────────────
-
+/// Euclidean distance between two P² homogeneous points (dehomogenises both).
 inline double euclideanDistanceP2(const Eigen::Vector3d& p_h,
                                   const Eigen::Vector3d& q_h) {
     Eigen::Vector2d p = p_h.head<2>() / p_h(2);
@@ -57,11 +54,9 @@ inline double euclideanDistanceP2(const Eigen::Vector3d& p_h,
 
 // ── Direct Euclidean isometry from two point-frames ──────────────────────────
 
-// Build the 3×3 projective matrix that represents the coordinate frame
-// anchored at p0 with p1 defining the positive x-direction.
-// Euclidean case: columns are [dehom(p0), unit_dir(p0→p1), perp_dir].
-//
-// Template parameter S allows float / double / long double.
+/// Build the 3×3 projective frame matrix anchored at `p0` with `p1`
+/// defining the positive x-direction (Euclidean case).  Columns:
+/// `[dehom(p0), unit_dir(p0→p1), perp_dir]`.
 template <typename S>
 Eigen::Matrix<S, 3, 3> makeFrameMatrix(Eigen::Matrix<S, 3, 1> p0_h,
                                         Eigen::Matrix<S, 3, 1> p1_h) {
@@ -84,11 +79,9 @@ Eigen::Matrix<S, 3, 3> makeFrameMatrix(Eigen::Matrix<S, 3, 1> p0_h,
     return M;
 }
 
-// Find the 3×3 Euclidean isometry (as a projective matrix) that maps
-// the frame (s1, s2) to the frame (t1, t2).
-//
-// Corresponds to Java P2.makeDirectIsometryFromFrames(s1, s2, t1, t2, Pn.EUCLIDEAN)
-// and P2Big.makeDirectIsometryFromFrames(...) (the BigDecimal / high-precision variant).
+/// 3×3 Euclidean isometry (as a projective matrix) that maps the
+/// frame `(s1, s2)` to the frame `(t1, t2)`.  Same as Java
+/// `P2.makeDirectIsometryFromFrames(..., Pn.EUCLIDEAN)`.
 template <typename S>
 Eigen::Matrix<S, 3, 3> makeDirectIsometryFromFramesEuclidean(
     Eigen::Matrix<S, 3, 1> s1, Eigen::Matrix<S, 3, 1> s2,