- profile key extra_drive: optional read-only second virtio-blk so an
overlay can ship guest-side shim libraries (/dev/vdb)
- SENS <name> protocol op: canvas-fed named values (built-in sensors /
buttons) served from PiInstance.sensor_state, pushed by the frontend
via the new pi_sensor_state WS message
- DISP <b64> protocol op: guest display commands forwarded to the
frontend as 'display' events (built-in screens)
- RaspberryPi3Bridge: onDisplay / onGpioPwm callbacks + setSensorState
- SimulatorCanvas hands piFamily boards their Pi bridge in
attachBuiltins (was ESP32-only)
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