refactor(usb): the configuration-descriptor walk moved out of the event loop, unchanged

xhci.c's XHCI_XFER_CONFIG_DESC_FULL case walked the descriptor set inline, in
the event loop, where nothing could test it. The walk is now
usb_config_parse() (kernel/src/usb/usb_config.c), returning what it found;
the case acts on it as before -- the same messages, the same decisions: the
first Interface descriptor only, a BOT interface's bulk endpoints (the last of
each direction), a boot keyboard's first interrupt IN endpoint, bLength 0
ending the walk, nothing read past len. One difference, in debug output only:
the bulk endpoints are logged IN then OUT, not in descriptor order.

kernel/tests/test_usb_config.c, 7 checks, pins the walk as moved: a BOT stick
as QEMU presents it, a boot keyboard with its HID descriptor, a mouse before a
drive (the first interface only), a drive with bulk IN only, bLength 0, an
endpoint cut by len, an interface cut before its protocol byte. All pass.
Built for amd64, aarch64, riscv64; the boots come with the protocol (a data
stick enumerates through this path). The fixes for #170 and #171 follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017RzZgrf7V7Zk2iczBXDRbm

kernel/include/starkernel/usb_config.h

@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: LicenseRef-Starship-1.0
+ * Copyright (c) 2023-2026 Robert A. James
+ */
+/* usb_config.h -- a USB configuration descriptor set, read: what xhci.c's event loop needs to know
+ * of a device (its first interface, and that interface's endpoints). Moved out of the loop's
+ * XHCI_XFER_CONFIG_DESC_FULL case unchanged, so it can be tested on the host
+ * (kernel/tests/test_usb_config.c; docs/v4.0.0/plans/2026-11-12-kernel-host-tests.md).
+ */
+#ifndef STARKERNEL_USB_CONFIG_H
+#define STARKERNEL_USB_CONFIG_H
+
+#include <stdint.h>
+
+#define USB_CONFIG_NO_INTERFACE 0   /* no Interface descriptor in the set */
+#define USB_CONFIG_MSC          1   /* Mass Storage / SCSI / Bulk-Only Transport */
+#define USB_CONFIG_KBD          2   /* a HID boot keyboard */
+#define USB_CONFIG_OTHER        3   /* anything else -- not used */
+
+typedef struct {
+    int      kind;                                /* USB_CONFIG_* */
+    uint8_t  iface_class, iface_subclass, iface_protocol;
+    uint8_t  config_value;                        /* bConfigurationValue, for SET_CONFIGURATION */
+    uint8_t  bulk_in_ep, bulk_out_ep;             /* USB_CONFIG_MSC: 0 when not found */
+    uint16_t bulk_in_max, bulk_out_max;
+    uint8_t  hid_iface;                           /* USB_CONFIG_KBD */
+    uint8_t  hid_ep, hid_interval;                /* hid_ep 0 when not found */
+    uint16_t hid_max;
+} usb_config_t;
+
+/* Read the first `len` bytes of a configuration descriptor set (the Configuration descriptor and
+ * the Interface and Endpoint descriptors after it). Only the first Interface descriptor counts;
+ * a descriptor with bLength 0 ends the walk. Never reads past `len`. */
+void usb_config_parse(const uint8_t *desc, uint16_t len, usb_config_t *out);
+
+#endif