Look up CSR neighbours with async, batched calls #74

Problem

Expansion reads neighbours with a synchronous call on an in-memory slice (neighbors_from). A disk-backed or partially loaded adjacency can't sit behind that interface without blocking a runtime thread on every lookup.

Idea

Look up the neighbours for a chunk of input rows with one async, batched call ("neighbours for these N keys"), then run today's loop over the result, so reads can be grouped. The review of lance-format/lance-graph#162 asks for async, version-aware adjacency and materialization interfaces for the same reason.

Status

Not needed by #72, which keeps the CSR loaded whole in memory. It becomes a prerequisite only if partial or disk-backed loading becomes a goal.