Add ATT/GATT protocol types to btusb hci.rs: AttPdu with 8 builder
methods, GattService/GattCharacteristic discovery types, ATT response
parsers, ATT-over-ACL L2CAP helpers. 12 new tests (137 total btusb).
Migrate i2c-hidd from legacy ProducerHandle to InputProducer with
named producer fallback (i2c-hid), completing U3 driver migrations.
Update BLUETOOTH-IMPLEMENTATION-PLAN.md with B1/B2 completion evidence,
exit criteria assessment, and updated support language.
Add complete HCI protocol module (hci.rs) with packet types, 55+ constants,
command builders (Reset, Read BD Addr, Read Local Version, LE scan/connect),
event parsers, and structured result types. Add USB transport abstraction
(usb_transport.rs) with UsbHciTransport trait and StubTransport for testing.
Wire btusb daemon with endpoint descriptor parsing, HCI init sequence
(Reset → Read BD Addr → Read Local Version), ControllerState state machine,
and enhanced status output. Replace all expect()/unwrap() calls in btctl
and wifictl with proper error handling and graceful fallback.
91 btusb tests, 27 btctl tests, 2 wifictl tests passing.
vasilito