feat(phase4a+4b): Newton solver + CGAL mesh I/O

Phase 4a — newton_solver.hpp:
  - newton_euclidean(): SimplicialLDLT on H (PSD); solves H·Δx = −G
  - newton_spherical(): SimplicialLDLT on −H (NSD→PSD); solves (−H)·Δx = G
  - Backtracking line search (halving α up to 20×) for global convergence
  - NewtonResult struct: x, iterations, grad_inf_norm, converged
  - 7 tests: 4 spherical (convergence, few iters, large perturbation,
    field consistency) + 3 Euclidean (triangle pinned, quad pinned,
    mixed pinned — all with natural-theta equilibrium at x=0)

Phase 4b — mesh_io.hpp:
  - read_mesh() / write_mesh(): CGAL::IO::read/write_polygon_mesh wrappers
  - load_mesh() / save_mesh(): throwing convenience versions
  - Supports OFF, OBJ, PLY (format detected by file extension)
  - 6 tests: OFF round-trip (tet + quad), OBJ round-trip, missing-file throw,
    vertex-position preservation, save/load convenience wrappers

All 75 cgal tests pass (3 skipped as before).

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

code/tests/cgal/CMakeLists.txt

@@ -24,6 +24,12 @@ add_executable(conformallab_cgal_tests
     # ── Phase 3f: Hessians (cotangent Laplacian, spherical + Euclidean) ───
     test_euclidean_hessian.cpp
     test_spherical_hessian.cpp
+
+    # ── Phase 4a: Newton solver ────────────────────────────────────────────
+    test_newton_solver.cpp
+
+    # ── Phase 4b: Mesh I/O (CGAL::IO) ─────────────────────────────────────
+    test_mesh_io.cpp
 )
 
 target_include_directories(conformallab_cgal_tests SYSTEM PRIVATE