MeshCore: adaptive airtime + channel monitoring

[M0LTE]

Parent: #137. High interest. Be a dynamically good citizen, not just statically capped.

Monitor channel occupancy — LOG_RX_DATA (0x88) frames the companion already emits, plus CAD / RSSI — to estimate how busy the shared channel is, and adaptively tighten the airtime budget and inter-packet pacing when the channel is busy (listen-before-talk style). Feeds the airtime governor.

[M0LTE]

Implemented in #162 (merged d8c794e): ChannelMonitor estimates occupancy from LOG_RX_DATA (0x88); the bearer does listen-before-talk + congestion backoff (with per-node fairness jitter), configurable via DAPPS_MESHCORE_CONGESTION_BACKOFF_FRACTION / _LBT_GUARD_MS. Validated on air (backoff at ~21% occupancy).