Zigbee.Adapter backend for Silicon Labs EmberZNet dongles (EZSP over ASH).

Owns a Zigbee.EZSP connection, runs the EmberZNet-specific coordinator sequence (config, security/keys, endpoints, policies, form/reestablish, unicast), and normalizes the NCP's unsolicited callbacks into the backend-neutral events Zigbee.Adapter promises:

{:zigbee, :device_joined, %{node_id: _, eui64: _}}   # trustCenterJoinHandler
{:zigbee, :message, %Zigbee.Message{}}                # incomingMessageHandler

Start it through the Zigbee facade:

{:ok, zb} = Zigbee.start_link(Zigbee.EZSP.Adapter, device: "/dev/ttyACM0", speed: 460_800)

Tests may inject a pre-started EZSP process with the :ezsp option instead of a serial :device (see Zigbee.FakeEZSP).