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/main.c

@@ -0,0 +1,23 @@
+// Simulator: speaks CTAPHID over stdin/stdout, one 64-byte packet at a time.
+#include "ctaphid.h"
+#include "host.h"
+#include "pin.h"
+#include "platform.h"
+#include "presence.h"
+#include "storage.h"
+
+#define TICK_MS 50
+
+int main(void) {
+    uint8_t packet[HID_PACKET_SIZE];
+
+    platform_init();
+    storage_init();
+    pin_init();
+    ctaphid_init();
+    for (;;) {
+        if (host_receive(packet, TICK_MS)) ctaphid_handle_packet(packet);
+        ctaphid_tick();
+        presence_tick();
+    }
+}