Artemis Milestone 2b complete: xHCI PCI discovery and BAR0 mapping

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>

include/starkernel/xhci_driver.h

@@ -0,0 +1,42 @@
+/*
+ * xhci_driver.h — xHCI USB host controller driver public API for StarKernel
+ *
+ * Register-layout definitions live in xhci.h; this header is the driver's
+ * own state and public entry points, matching virtio_blk.h's split.
+ */
+
+#ifndef STARKERNEL_XHCI_DRIVER_H
+#define STARKERNEL_XHCI_DRIVER_H
+
+#include <stdint.h>
+#include "starkernel/pci.h"
+#include "starkernel/xhci.h"
+
+/* Driver state for one xHCI controller instance. Only one controller is
+ * supported (matches virtio_blk's single-device precedent). */
+typedef struct {
+    PciDevice          pci;
+    uint64_t           bar0_phys;   /* physical MMIO base, BAR0 */
+    xhci_cap_regs_t    *cap;        /* BAR0 + 0 */
+    xhci_op_regs_t     *op;         /* BAR0 + cap->cap_length */
+    xhci_runtime_regs_t *runtime;   /* BAR0 + cap->rts_off */
+    xhci_doorbell_t     *doorbell;  /* BAR0 + cap->db_off */
+    uint32_t            max_slots;
+    uint32_t            max_ports;
+    uint32_t            max_intrs;
+} xhci_dev_t;
+
+/*
+ * xhci_find_and_map — locate the xHCI controller on PCI bus 0, enable it
+ *                     (I/O+MEM+bus-master), map its BAR0 MMIO region, and
+ *                     fill in the four register-region pointers in *dev.
+ *
+ * dev must point to a zero-initialised xhci_dev_t.
+ *
+ * Returns  0 on success.
+ * Returns -1 if no xHCI device was found on the PCI bus.
+ * Returns -2 if the BAR0 mapping failed.
+ */
+int xhci_find_and_map(xhci_dev_t *dev);
+
+#endif /* STARKERNEL_XHCI_DRIVER_H */