a1fdf9782b
Add GATT client helpers to btusb scheme (GattDiscoverServices, GattDiscoverChars, GattReadChar, GattServices, GattCharacteristics) with ATT-over-ACL transport. Wire HciBackend::read_char to perform real GATT workflow through scheme filesystem (discover services → discover characteristics → read value) instead of hardcoded stub. 209 tests passing (151 btusb + 56 btctl + 2 wifictl).