qector-pymatching-compat

The PyMatching-compatible shim in qector-decoder-v3 (manual 17.1). Covers qector_decoder_v3.pymatching_compat.Matching as a drop-in for pymatching.Matching, the supported methods (from_check_matrix, from_detector_error_model, add_edge, add_boundary_edge, decode, decode_batch, decode_to_edges_array), the fact that it is backed by the exact Blossom decoder, the one-line import swap, and the smoke test pattern via…