MeshCore: backhaul seam adjustments for a broadcast datagram bearer

[M0LTE]

Parent: #137. The backhaul seam is the right shape, but a broadcast PSK channel needs three adjustments (from the fresh-look review):

1. Broadcast-route model. A private channel is one shared medium, but OutboundMessageManager floods one SendAsync per neighbour route (OutboundMessageManager.cs:208–230) — N transmissions where one suffices. Model a first-class broadcast route (one send to the channel; addressee self-selects via IsLocal) or coalesce identical (msgId, channel) sends.
2. Positive bearer selection. Replace Dappsv1SessionBackhaul.CanHandle's route.UdpEndpoint is null catch-all with an explicit route.Bearer discriminator, so adding MeshCore doesn't force editing AGW's predicate.
3. Airtime on the data path. AirtimeAccountant.TryReserve is discovery-only today; wire it into the MeshCore SendAsync / data-forward path.

[M0LTE]

Implemented in #162 (merged d8c794e): additive DbNeighbour.MeshCoreChannel copied through RouteBuilder, so a neighbour with a channel hint is selected by the MeshCore bearer (positive selection).