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/euclidean_functional.hpp

@@ -162,7 +162,7 @@ inline void compute_euclidean_lambda0_from_mesh(ConformalMesh& mesh, EuclideanMa
         if (len > 1e-15)
             m.lambda0[e] = 2.0 * std::log(len);
         else
-            m.lambda0[e] = -30.0;   // degenerate edge
+            m.lambda0[e] = LOG_EDGE_LENGTH_FLOOR;   // degenerate edge
     }
 }