Add op_system metadata passthrough to op_engine connector #10
Description
Mixing kernels now flow, but operator specs and any axis-aware operator metadata from op_system aren’t forwarded to op_engine.
Work: build ModelCore using axis sizes/names/coords from op_system meta (n_subgroups + other axes); merge operators from meta (with config override) into OperatorSpecs; set operator_axis from meta; validate operator shapes vs axis length; keep mixing merge.