AncillaryCaps interface
Capabilities for an ST 2110-40 ancillary receiver. media_types is effectively pinned to video/smpte291. The subtitles / ancillary fields pick which VANC payload formats the receiver decodes; the per-DID/SDID dispatch happens server-side. The defaults from defaultAncillaryMediaType() cover the most common case (SCTE-104 only).
Signature:
export interface AncillaryCaps