DataLayerHeartbeatSender
Centralized heartbeat sender for the data layer.
Consolidates heartbeat nonce management into a single monotonically increasing counter, preventing the firmware's per-connection duplicate-write filter (byte-level memcmp) from silently dropping consecutive heartbeats.
This is distinct from org.meshtastic.core.network.transport.HeartbeatSender, which operates at the transport layer with raw byte encoding. This class works at the protobuf/data layer through PacketHandler.