Analysis comment: Arduino GPIO Read on output pins returns stale Low

[Copilot]

Reported on Arduino Uno R3 + ConfigurableFirmata 3.3: after pin.Toggle() the LED turns on, but pin.Read() still returns Low.

No code changes — per the agent instruction, this PR records an analysis of whether Toggle should work on this board.

Findings (src/devices/Arduino/ArduinoGpioControllerDriver.cs)

• Toggle works, even without a prior Write. OpenPin seeds _pinValues[pin] = Low, and Toggle is implemented as Write(pin, !_pinValues[pin]), so the first toggle correctly drives the pin High.
• Read on an output pin is broken. Read always delegates to _device.ReadDigitalPin, which returns the value sampled from the digital input port. ConfigurableFirmata only auto-reports DIGITAL_MESSAGE for ports that have at least one input-configured pin; on a pure-output port the cached port stays 0, so Read() returns Low regardless of what was last written.
• This violates the documented GpioPin.Read() contract: "If the pin is configured as an output, this value is the last value written to the pin."

Suggested fix (not applied here)

Short-circuit Read for output pins to return the cached written value:

protected override PinValue Read(int pinNumber)
{
    if (_pinModes.TryGetValue(pinNumber, out var mode) && mode == PinMode.Output)
    {
        return _pinValues[pinNumber];
    }

    _pinValues[pinNumber] = _device.ReadDigitalPin(pinNumber);
    return _pinValues[pinNumber];
}

Happy to follow up with a separate PR implementing this if reviewers agree.

[raffaeler]

@pgrawehr you may like this or not, decision is yours ...