Expand description
CooBuilder — a small HashMap backed coordinate format accumulator.
Deduplicates (i, j) entries on insert (each add is O(1) amortized,
independent of nnz). Replaces the previous Vec linear scan
accumulator, which was O(nnz²) per case.
Square by default (new), rectangular via new_rect for the incidence,
flow map, and generator→bus matrices.