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.
vasilito