board/io4.c: Add initial emulator for USB-mode IO-4

board/io4.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <windows.h>
+
+#include <stdint.h>
+
+enum {
+    /* System buttons in button[0] */
+
+    IO4_BUTTON_TEST     = 1 << 9,
+    IO4_BUTTON_SERVICE  = 1 << 6,
+};
+
+struct io4_state {
+    uint16_t adcs[8];
+    uint16_t spinners[4];
+    uint16_t chutes[2];
+    uint16_t buttons[2];
+};
+
+struct io4_ops {
+    HRESULT (*poll)(void *ctx, struct io4_state *state);
+};
+
+HRESULT io4_hook_init(const struct io4_ops *ops, void *ctx);