The Joystick API (/dev/input/js*) numbers buttons depending on driver
and connection type. Xbox One controllers via Bluetooth with a Share
button shifted X from button number 2 to 3, silently breaking the
mapping.
The evdev interface provides standardized BTN_* codes (BTN_SOUTH=A,
BTN_NORTH=X, etc.) that remain consistent across Xbox, PlayStation,
and Nintendo controllers over both USB and Bluetooth.
js* devices remain the discovery path, while /sys/class/input/jsN is
used to resolve the corresponding event* device. If the /sys lookup
fails, JoystickFallbackReader falls back to the old Joystick API.
