Progress endpoint (see module docs).

{costing, stateless: true, plan: {length_m, source, duration_s, distance_m}, position: {input, snapped, method, along_m}, progress: {fraction, travelled_m, plan_remaining_m}, off_route, off_route_m, off_route_threshold_m, arrived, remaining: {distance_m, duration_s, method}, eta: {duration_s, arrival_estimate_utc, traffic}}. method is projection (single fix, geometric) or map_matched (a recent_trace was matched through the engine). remaining is a fresh engine search from where the vehicle is to the plan's final point, so it can differ from progress.plan_remaining_m; eta.traffic carries the same coverage/confidence provenance POST /route reports