repo_flow binding_station is the argmin of the station bounds (flow model eq 2/3/5/6) #6068

repo_flow::sim_block names the binding station with fixed 0.85/0.95 thresholds; pr_flow already uses the argmin of the bounds. Make repo_flow use it. Falsifiers: L = cap reports cap; N-starved reports supply; a run with no merge queue never reports merge_queue. Part of epic #5510.