A leg-aware sink endpoint: an OPEN map from channel ROLE to the physical sink port that leg
feeds. A present L routes the bus's LEFT leg to that port; a present R the RIGHT leg. At
least one entry must be present. Stereo destination = {L, R}; a mono leg = {L} OR {R}.
Deliberately a role map (not a positional array / two fixed fields) so an R-only leg round-trips
distinctly and a future role (C / LFE / …) is added as another key with zero migration.
A leg-aware sink endpoint: an OPEN map from channel ROLE to the physical sink port that leg feeds. A present
Lroutes the bus's LEFT leg to that port; a presentRthe RIGHT leg. At least one entry must be present. Stereo destination ={L, R}; a mono leg ={L}OR{R}. Deliberately a role map (not a positional array / two fixed fields) so an R-only leg round-trips distinctly and a future role (C/LFE/ …) is added as another key with zero migration.