feat(p1): CLI extensions + quality measures + stereographic layout

Implement Phase-Session P1 quick wins (4 independent additions):

9h.1: Add --tol and --max-iter CLI options to conformallab_core
  - Newton solver tolerance [default 1e-8]
  - Newton iteration limit [default 200]
  - Thread both through run_euclidean / run_spherical / run_hyper_ideal
  - Update CLI parameter table in documentation

9h.2: Add -g cp_euclidean and -g inversive_distance geometry routes
  - run_cp_euclidean() & run_inversive_distance() pipelines (~60 lines each)
  - Face-based DOF assignment for CP-Euclidean
  - Vertex-based DOF assignment for Inversive-Distance
  - Both integrated into CLI geometry validator (IsMember)

9g.1: Create conformal_quality.hpp with validation measures
  - IsothermicityMeasure: metric anisotropy (conformality deviation)
  - DiscreteConformalEquivalenceMeasure: length-cross-ratio residuals
  - FlippedTriangles: detects inverted/degenerate triangles
  - LengthCrossRatio: discrete conformal invariant computation
  - ConvergenceUtility: aggregated convergence statistics (max/mean/sum)
  - Ported from Java: plugin/visualizer + convergence utilities
  - Includes sanity tests validating finite outputs on valid layouts

9d.3: Create stereographic_layout.hpp for S² → ℂ projection
  - Stereographic projection from north pole: S² → ℂ ∪ {∞}
  - Inverse projection: ℂ → S² for round-trip validation
  - Möbius centring: centres the 2-D point cloud at origin
  - stereographic_layout(Layout3D) -> Layout2D conversion
  - Round-trip tests: south pole, equator, random sphere points
  - Tests: projection/inverse consistency, north pole handling

Test results: 336/336 CGAL tests pass (272 pre-existing + 64 new from all phases)
- conformal_quality.cpp: 13 new tests (measures, isothermic, dce, convergence)
- stereographic_layout.cpp: 10 new tests (projection, inverse, round-trip, layout)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

code/include/gauss_bonnet.hpp

@@ -44,7 +44,7 @@
 //   double gauss_bonnet_rhs(mesh)            — 2π · χ(M)
 //   double gauss_bonnet_deficit(mesh, maps)  — lhs − rhs  (0 = satisfied)
 //   void   check_gauss_bonnet(mesh, maps [, tol])   — throws if violated
-//   void   enforce_gauss_bonnet(mesh, maps)  — shifts θ_v by uniform Δ
+//   double enforce_gauss_bonnet(mesh, maps)  — shifts θ_v by uniform Δ; returns |deficit|
 //   (HyperIdealMaps overloads are deleted — see box above)
 
 #include "conformal_mesh.hpp"
@@ -162,11 +162,16 @@ inline void check_gauss_bonnet(const ConformalMesh& mesh,
 // After this call, check_gauss_bonnet() will not throw (up to floating-point).
 // Modifies ALL vertices' θ_v (no v_idx filtering) — the shift is a property
 // of the target angles, independent of which vertices are free DOFs.
+//
+// H3 (test-coverage audit, 2026-06-01): both overloads now return the total
+// absolute correction applied: |Σ(2π−Θ_v) − 2π·χ|.  A large value signals
+// that the input angles were far from satisfying Gauss–Bonnet.
 
 /// Distribute the Gauss-Bonnet deficit uniformly across all `Θ_v`:
 /// add `δ = (lhs − rhs) / V` to every entry so that the identity holds
 /// exactly afterwards.  Overload for a raw property map.
-inline void enforce_gauss_bonnet(
+/// Returns `|lhs − rhs|` (total absolute correction applied).
+inline double enforce_gauss_bonnet(
     ConformalMesh&                                           mesh,
     ConformalMesh::Property_map<Vertex_index, double>&       theta)
 {
@@ -177,15 +182,17 @@ inline void enforce_gauss_bonnet(
     double delta = (lhs - rhs) / static_cast<double>(mesh.number_of_vertices());
     for (auto v : mesh.vertices())
         theta[v] += delta;
+    return std::abs(lhs - rhs);
 }
 
 /// Distribute the Gauss-Bonnet deficit uniformly across `maps.theta_v`.
 /// Supported for EuclideanMaps and SphericalMaps only.
 /// HyperIdealMaps overload is deleted — see header comment for why.
+/// Returns `|lhs − rhs|` (total absolute correction applied; see raw-map overload).
 template <typename Maps>
-inline void enforce_gauss_bonnet(ConformalMesh& mesh, Maps& maps)
+inline double enforce_gauss_bonnet(ConformalMesh& mesh, Maps& maps)
 {
-    enforce_gauss_bonnet(mesh, maps.theta_v);
+    return enforce_gauss_bonnet(mesh, maps.theta_v);
 }
 
 // enforce_gauss_bonnet for HyperIdealMaps is intentionally DELETED.