broadcastResponse property
What the broadcaster was told, as ARC's wire status name
(SEEN_ON_NETWORK, REJECTED, ...), or null when the broadcast was
recorded without an answer to go with it.
Nullable since bead libspiffy-f0sj: this used to be a non-null String
that the aggregate filled with the constant 'broadcast_success',
because BroadcastTransactionCommand had no field to carry ARC's real
answer. Events written before that change replay unaltered and still
read 'broadcast_success'; they are not evidence of anything ARC said.
Implementation
final String? broadcastResponse;