sendHeartbeat

Sends a heartbeat to the radio.

The firmware responds to heartbeats by queuing a queueStatus FromRadio packet, proving the link is alive and keeping the local node's lastHeard timestamp current.

Repeated rejections while disconnected are expected (e.g. a radio that left BLE range), so only the first HeartbeatRejectionLogPolicy.DEFAULT_WARN_LIMIT consecutive rejections log at warn severity; later ones are demoted to debug until a successful send resets the streak.