Implements a VHCI driver on top of the BCE transport:
- Virtual USB bus registration via usb_controller
- Port discovery and device enumeration
- Control, interrupt, and bulk endpoint support
- Firmware event handling with taskqueue
- Suspend/resume via BCE mailbox
Provides keyboard, trackpad, and Touch Bar access on T2 Macs.
Tested-on: MacBookPro16,2 (A2251), Mac mini 8,1 (A1993)