feat(usb): every xHCI controller is driven, not only the first

boards/BRINGUP.md A1, FABRIC-3 SXLII.  xhci.c keeps a table of controllers
(up to 8) and services each; kernel_main brings up index 0, 1, ... until
there is none.  repl.c's and blkio_usb.c's per-slot state is per
controller now (a slot ID is a controller's own), and each USB keyboard
keeps its own last report and lights; an unplugged keyboard releases what
it held.

Test first: keyboard and identity on a second controller behind a bridge,
Zuse on the first -- before, only Zuse; after, all three, on all three
ISAs.  Hot-plug and unplug on the second controller.  Acceptance unchanged.

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

kernel/include/starkernel/usb_kbd.h

@@ -20,25 +20,27 @@
 #include <stdint.h>
 
 /* One boot report: byte 0 the modifier bits, byte 1 reserved, bytes 2-7
- * up to six keys held (HID usage IDs). `len` below 3 is ignored. */
-void usb_kbd_report(const uint8_t *report, uint32_t len);
+ * up to six keys held (HID usage IDs). `len` below 3 is ignored. `prev`
+ * is that keyboard's own last report, 8 bytes, kept by the caller -- each
+ * keyboard is compared with itself, never with another. */
+void usb_kbd_report(uint8_t prev[8], const uint8_t *report, uint32_t len);
 
-/* Forgets the keys held: the keyboard has gone. */
-void usb_kbd_reset(void);
+/* The keyboard has gone: every key it still held is released, and its
+ * last report forgotten -- no Shift stays down after it. */
+void usb_kbd_forget(uint8_t prev[8]);
 
 /* The next key event: 1 with *keycode (Linux) and *pressed (1/0), or 0. */
 int usb_kbd_pop_event(uint16_t *keycode, int *pressed);
 
 /* The lock lights (boot output report bits, HID 1.11 appendix B.1). The
- * console says which locks are on; the xHCI driver sends them to the
+ * console says which locks are on; the xHCI driver sends them to each
  * keyboard with SET_REPORT whenever they differ from what it last sent. */
 #define USB_KBD_LED_NUM    0x01u
 #define USB_KBD_LED_CAPS   0x02u
 #define USB_KBD_LED_SCROLL 0x04u
 void usb_kbd_set_leds(uint8_t leds);
-/* 1 and the lights to send if they changed since last sent, else 0. */
-int usb_kbd_leds_pending(uint8_t *leds);
-/* The driver has sent these. */
-void usb_kbd_leds_sent(uint8_t leds);
+/* The lights every keyboard should show. Each keyboard's driver state
+ * holds what that keyboard was last sent. */
+uint8_t usb_kbd_leds_wanted(void);
 
 #endif /* STARKERNEL_USB_KBD_H */