feat(usb): a USB keyboard -- a full one -- and no hang on a PC without PS/2

boards/BRINGUP.md A3 and S3, FABRIC-3 SXLI.  A HID boot keyboard is
configured (interrupt IN endpoint, SET_PROTOCOL boot) and its reports
become the same Linux-keycode events as PS/2 and virtio (usb_kbd.c): the
full layout -- F-keys, navigation block, arrows, Num Lock, keypad.  The
console treats Caps and Num Lock as toggles shown on the keyboard's
lights, and the keypad as digits or navigation.  While a USB keyboard
runs, its event ring is serviced every tick, not once a second.

Found on the way: with no i8042 (port 0x64 reads 0xFF) the kernel hung at
boot in an endless drain -- a legacy-free PC would stop there.  Detected
and skipped now; drains bounded.

QEMU, amd64 with the USB keyboard the only one: 1 2 + . on main keys and
keypad, Caps Lock, Num Lock all right; aarch64/riscv64 configure it too.
Acceptance on three ISAs unchanged; identities hot-plug.

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

kernel/include/starkernel/xhci.h

@@ -279,6 +279,14 @@ typedef struct {
 #define USB_CLASS_MASS_STORAGE       0x08u
 #define USB_SUBCLASS_SCSI            0x06u  /* SCSI transparent command set */
 #define USB_PROTOCOL_BOT             0x50u  /* Bulk-Only Transport */
+#define USB_IFACE_OFF_NUMBER         2u     /* bInterfaceNumber */
+/* A boot-protocol keyboard (HID 1.11 section 4): the class a BIOS can use,
+ * and what this driver reads keys from (2026-10-14, boards/BRINGUP.md A3). */
+#define USB_CLASS_HID                0x03u
+#define USB_SUBCLASS_HID_BOOT        0x01u
+#define USB_PROTOCOL_HID_KEYBOARD    0x01u
+#define USB_HID_REQ_SET_PROTOCOL     0x0Bu  /* class, interface recipient */
+#define USB_HID_PROTOCOL_BOOT        0u
 
 /* Standard USB Endpoint descriptor field offsets (7 bytes, USB 2.0 spec
  * table 9-13) -- found the same way as the Interface descriptor: walked
@@ -292,6 +300,8 @@ typedef struct {
 #define USB_EP_ADDR_NUM_MASK          0x0Fu
 #define USB_EP_ATTR_TYPE_MASK         0x03u
 #define USB_EP_TYPE_BULK              0x02u
+#define USB_EP_TYPE_INTERRUPT         0x03u
+#define USB_EP_OFF_INTERVAL           6u  /* bInterval */
 
 /* Bulk-Only Transport Command Block Wrapper (USB Mass Storage Class Bulk-
  * Only Transport spec, section 5.1) -- sent host-to-device on the bulk OUT
@@ -400,6 +410,10 @@ typedef struct {
 #define XHCI_EVT_SLOT_ID(control)         (((uint32_t)(control) >> 24) & 0xFFu)
 #define XHCI_COMPLETION_CODE_SUCCESS      1u
 #define XHCI_COMPLETION_CODE_STALL_ERROR  6u
+#define XHCI_COMPLETION_CODE_SHORT_PACKET 13u
+/* Transfer Event: the Endpoint ID (DCI) the transfer was on, bits 20:16 of
+ * the control word (xHCI 1.2 table 6-38). */
+#define XHCI_EVT_EP_ID(control)           (((uint32_t)(control) >> 16) & 0x1Fu)
 
 /* Port Status Change Event TRB layout (xHCI 1.2 spec table 6-34):
  * parameter[31:24] = Port ID (1-based, matches PORTSC array indexing
@@ -440,6 +454,8 @@ typedef struct {
 #define XHCI_EP_CTX_TYPE_CONTROL_BIDI  4u  /* EP0 */
 #define XHCI_EP_CTX_TYPE_BULK_OUT      2u  /* xHCI 1.2 spec table 6-9 */
 #define XHCI_EP_CTX_TYPE_BULK_IN       6u
+#define XHCI_EP_CTX_TYPE_INTERRUPT_IN  7u
+#define XHCI_EP_CTX_INTERVAL_SHIFT     16 /* dword0: period 2^Interval x 125 us */
 #define XHCI_EP_CTX_CERR_SHIFT         1
 #define XHCI_EP_CTX_MAX_PACKET_SHIFT   16