Fix motif lifecycle and edgelist ingest

[j6k4m8]

This cleans up a few rough edges around replacing, saving, loading, and ingesting motifs.

• makes from_nx() fully replace old parsed state and work with multigraph edges
• keeps caller-owned streams open while still closing files opened from paths
• accepts path-like objects in save/load
• applies endpoint dtypes consistently for file and DataFrame inputs
• rejects missing endpoint IDs instead of turning them into bogus graph nodes