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>

udev/69-skey-snap.rules

@@ -0,0 +1,7 @@
+# Lets snap-packaged Firefox use the Skey security key.
+#
+# snapd only lets snaps open FIDO keys whose USB IDs are on a fixed list, and
+# Skey's pid.codes test ID isn't on it. This adds the tag snapd looks for. The
+# file must sort before 70-snap.firefox.rules, so that snapd's own rule sees
+# the tag and grants access as soon as the key is plugged in.
+SUBSYSTEM=="hidraw", KERNEL=="hidraw*", ATTRS{idVendor}=="1209", ATTRS{idProduct}=="0001", TAG+="snap_firefox_firefox"