Run shortestPath from many sources with multi-source BFS #76

Problem

shortestPath runs one breadth-first search per source node (a source with one target searches from both ends; a source with several targets shares one forward search). A query that starts from many source nodes repeats traversals over the same parts of the graph.

Idea

Multi-source BFS runs many breadth-first searches over the CSR at once. Each search is a bit in a per-node bitset, so one pass over a neighbour list advances every search that has reached that node. DuckPGQ uses it for path-finding (CIDR 2023 paper, cited in lance-format/lance-graph#159).