MeshCore H1: binary Companion bearer (IDappsBackhaul / IBackhaulInbox)

[M0LTE]

Parent: #137. Implement the MeshCore Companion-over-USB bearer in DAPPS, validated by the standalone PoC in poc/MeshCorePoc/.

• MeshCoreCompanionBackhaul : IDappsBackhaul — carry BackhaulMessage over the binary channel-data path (SEND_CHANNEL_DATA 0x3E / CHANNEL_DATA_RECV 0x1B), data_type=0xFFFF, 1-byte nonce/flags header (the mesh dedups identical frames — binary has no on-air timestamp). Reuse BackhaulMessageCodec + Packetiser.
• Hosted inbound reader (MSG_WAITING 0x83 → SYNC_NEXT_MESSAGE 0x0A pull loop) → IBackhaulInbox.DeliverAsync; sender identity via LinkSourceCallsign with a MESHCORE sentinel fallback.
• zstd + versioned shared dictionary (collapses most messages to 1 packet; per-message compressed flag already in the wire header).
• App-layer reliability over an unacked flood: idempotency on dapps-id, end-to-end ack, TTL resends with long backoff.
• Discovery via B5 passive learning (no flooded beacons).

Wire-shape findings from the PoC (see poc/MeshCorePoc/README.md + CHARACTERISATION.md): firmware v1.16.0 returns the legacy 0x08 text frame (not v3 0x11) but binary 0x1B as expected; the text path auto-prepends "<name>: " (binary does not — verified byte-identical on air); ~165 B/packet; companion firmware can hang (needs a serial watchdog + reset).