feat(phase3c): port SphericalFunctional onto ConformalMesh; update README

New headers:
- spherical_geometry.hpp: spherical arc length l(λ) and half-angle formula
  for interior angles of a spherical triangle (SphericalFaceAngles struct)
- spherical_functional.hpp: SphericalMaps bundle, setup/assign DOF helpers,
  compute_lambda0_from_mesh(), gradient (Θ_v − Σα_v; Schläfli edge formula),
  energy via 10-point Gauss-Legendre path integral, gradient_check_spherical()

Updated:
- mesh_builder.hpp: add make_spherical_tetrahedron() (vertices on unit sphere)
  and make_octahedron_face() (single right-angled spherical triangle)
- tests/cgal/CMakeLists.txt: enable test_spherical_functional.cpp
- README.md: rewrite for CGAL-package goal, two test targets, all headers,
  updated project tree, Phase progress table, key design decisions

Tests (cgal.SphericalFunctional.*): 8 active + 1 skip
- OctaFaceAnglesAreRightAngles, SpherTetAngleSumExceedsPi
- GradientCheck_{OctaFaceVertex, SpherTetVertex, SpherTetAllDofs,
  SpherFan4Vertex, MixedPinnedVertices}
- AnglesFiniteAtKnownPoint
All 30 cgal.* tests pass (2 @Ignore skips).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/tests/cgal/CMakeLists.txt

@@ -15,8 +15,8 @@ add_executable(conformallab_cgal_tests
     # ── Phase 3b: HyperIdealFunctional ─────────────────────────────────────
     test_hyper_ideal_functional.cpp
 
-    # ── Phase 3c: SphericalFunctional   (to be added) ──────────────────────
-    # test_spherical_functional.cpp
+    # ── Phase 3c: SphericalFunctional ─────────────────────────────────────
+    test_spherical_functional.cpp
 )
 
 target_include_directories(conformallab_cgal_tests SYSTEM PRIVATE