Skey: a FIDO2 security key on the Raspberry Pi Pico

Firmware for the Pico / Pico 2 that acts as a USB FIDO2 authenticator:
CTAP2 and U2F, passkeys, BOOTSEL as the touch button, and clientPIN with
PIN/UV auth protocols 1 and 2. A host simulator runs the same core so the
python-fido2 test suite exercises it without hardware.

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

host/host.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "platform.h"
+
+// Reads one packet from stdin, waiting up to timeout_ms. Exits the process
+// when stdin is closed.
+bool host_receive(uint8_t packet[HID_PACKET_SIZE], int timeout_ms);