refactor(constants): centralize magic constants from functionals (N4/N6 audit)

- Add LOG_EDGE_LENGTH_FLOOR (-30.0) for degenerate edge handling
- Add HYPER_IDEAL_SCALE_FLOOR (0.01) for negative-scale clamping
- Add ASIN_DOMAIN_GUARD (1.0 - 1e-15) for asin argument bounding
- Each constant is documented with rationale and units
- Update 4 usage sites: euclidean_functional, hyper_ideal_functional,
  spherical_functional, spherical_geometry
- Add constants.hpp include to hyper_ideal_functional and spherical_functional

282/282 tests pass. Addresses N4 (unnamed magic constants) and N6 (centralize
tolerances) from numerical-stability audit.

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

code/include/spherical_functional.hpp

@@ -35,6 +35,7 @@
 // └──────────────────────────────────────────────────────────────────────────┘
 
 #include "conformal_mesh.hpp"
+#include "constants.hpp"
 #include "spherical_geometry.hpp"
 #include "gauss_legendre.hpp"
 #include <CGAL/boost/graph/iterator.h>
@@ -159,7 +160,7 @@ inline void compute_lambda0_from_mesh(ConformalMesh& mesh, SphericalMaps& m)
         if (half_sin > 1e-15)
             m.lambda0[e] = 2.0 * std::log(half_sin);
         else
-            m.lambda0[e] = -30.0;  // very short edge: essentially 0
+            m.lambda0[e] = LOG_EDGE_LENGTH_FLOOR;  // very short edge: essentially 0
     }
 }