BLE-MIDI transport: advertise the phone as a Bluetooth LE MIDI peripheral so a computer (or another phone) connects to it as a central and plays it.

This is the "phone keyboard drives a computer synth, wirelessly" path. It's a thin layer over MobBluetooth.Le (the generic GATT-peripheral primitive): the only MIDI-specific parts here are the two well-known BLE-MIDI UUIDs and the packet framing (a 13-bit-millisecond timestamp header per the BLE-MIDI spec). Everything underneath — advertising, the GATT server, notifications, incoming writes — is generic Bluetooth and lives in mob_bluetooth.

Once connected, the peripheral shows up in the host's MIDI stack (macOS: Audio MIDI Setup → Bluetooth; iOS/other: any BLE-MIDI client).

Sending (phone → computer)

socket = MobMidi.Ble.advertise(socket, "Mob MIDI")
# wait for {:bt_le, :subscribed, _} (the central enabled notifications)
ts = System.monotonic_time(:millisecond)
socket = MobMidi.Ble.send_note_on(socket, 0, 60, 100, ts)
socket = MobMidi.Ble.send_note_off(socket, 0, 60, 0, ts)

Receiving (computer → phone)

Incoming BLE-MIDI arrives as a MobBluetooth.Le write on the MIDI characteristic:

{:bt_le, :write, %{characteristic: char, bytes: packet}}

Strip the BLE framing with decode_packet/1, then MobMidi.parse/1:

def handle_info({:bt_le, :write, %{bytes: packet}}, socket) do
  for ev <- MobMidi.parse(MobMidi.Ble.decode_packet(packet)), do: react(ev)
  {:noreply, socket}
end