fix(usb): the xHCI controller is found by class on any bus, sized, taken from the firmware, and 64-byte contexts laid out

boards/BRINGUP.md A1, FABRIC-3 SXL.  It was matched by QEMU's own IDs on
bus 0 alone, so no real board's was found.  Now pci_find_class() reads
every bus/slot/function; pci_bar_size() sizes BAR0; the USB Legacy Support
capability hands the controller over from the BIOS, SMIs off; CSZ=1
contexts are placed 64 bytes apart instead of refused.  XHCI_QEMU_ARGS
lets QEMU place the controller as real hardware does.

Test first: an NEC controller behind a PCIe root port -- no attach before,
Zuse minted after, on all three ISAs.  Standard acceptance unchanged; five
hot-plugged identities attach.  Not rehearsable: firmware handoff, CSZ=1.
Still open: only the first controller found is used.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kernel/include/starkernel/xhci_driver.h

@@ -538,9 +538,8 @@ int xhci_cmd_disable_slot(xhci_dev_t *dev, uint32_t slot_id);
  * is servicing (xHCI 1.2 spec table 7-13 speed IDs) -- used to pick EP0's
  * default Max Packet Size before any device descriptor has been read.
  *
- * Refuses (-2) if HCCPARAMS1.CSZ indicates 64-byte contexts -- only
- * 32-byte contexts are implemented (see xhci.h's own doc comment on
- * xhci_slot_ctx32_t).
+ * Lays the contexts out 32 or 64 bytes apart, as HCCPARAMS1.CSZ says
+ * (xhci.c's xhci_ctx_bytes(); 64-byte contexts refused until 2026-10-14).
  *
  * Called from xhci_poll_events()'s Command Completion handling once Enable
  * Slot succeeds -- not called directly by other code.