v0.1.0 First version

Implements core functionality for periodic graphs on Z^d:

• PeriodicGraph (undirected) and PeriodicDiGraph (directed), enforcing one edge per (u, v, tvec).
• PeriodicMultiGraph and PeriodicMultiDiGraph when you really need parallel edges for the same (u, v, tvec).
• A PeriodicComponent view that exposes lattice invariants of the component translation subgroup:
  • rank (periodic dimension of the component),
  • torsion_invariants (torsion / interpenetration signature),
  • deterministic inst_key(...) keys for lifted instances within a component.
• same_fragment(...): exact "are these two lifted instances in the same connected fragment?" checks.
• shortest_path_quotient(...): fast BFS in the quotient with connectivity='directed'|'weak'.