Return the per-leg seat map (carriage → deck → layout grid) and/or free-assign preference groups for a locked train fare.

fareId is the Mercury UUID returned by POST /v1/trains/fares/build; Mercury reverse-maps it to the upstream provider identifier server-side.

A leg with no graphical map returns error set and an empty carriages list, and may still include preferences when the carrier offers free auto-assign options.

Provider selection handles are never returned on the wire.