graph-lite: CSR-style adjacency storage

[sebyx07]

Flat, contiguous arrays for node and edge data — chosen because it is cache-friendly to walk and maps almost directly onto the persisted layout, so a reload can be close to a read rather than a parse-and-rebuild.

Done when

• Node and edge data live in contiguous arrays; traversal walks them without pointer chasing.
• Both forward and reverse adjacency are materialised. Reverse traversal is a first-class operation, not something bolted on later — retrofitting it is a storage-format change.
• Insert and delete maintain both directions without invalidating live ids.
• A benchmark exists (even a crude one) so later claims about traversal cost are measured rather than asserted.

Reference

docs/graph-lite/README.md