feat(io): full VASP POSCAR spec support (load/save) + spec-compliance fixes
Summary
- Full POSCAR spec support: selective dynamics, velocities, lattice velocities (VASP6), predictor-corrector, scaling normalization, and a new
save(). - Fixed two verified spec-compliance bugs post-implementation: velocities/lattice-velocities section order and coordinate-loop overrun;
save()gated-property symmetry, a truncated-coordinate crash, and malformed-velocity-line detection.
Edited by Marek Białoruski