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>
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