New src/starkernel/usb/ subsystem directory (added to both
LOADER_SRCS_BASE and KERNEL_SRCS_BASE wildcards in Makefile.starkernel,
matching the existing virtio/*.c pattern). xhci_find_and_map() locates
the controller via the already-generic pci_find_first(), enables it,
maps BAR0 via the already-generic pci_map_bar(), and fills in all four
register-region pointers (cap/op/runtime/doorbell) plus max_slots/
max_ports/max_intrs from HCSPARAMS1 -- ready for controller bring-up
(2c) to consume directly.
No pci.c extension needed, per 2a's finding that PCI discovery here is
ID-based lookup (already generic), not class-code scanning. Verified:
clean standalone syntax check, full amd64 kernel build with zero
warnings, live boot still reaches POST 1012/0/0 unaffected (nothing
calls xhci_find_and_map() yet, so this is purely additive).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
