Implement GpioDriver on Pca95x4 for GpioController access

[Copilot]

Pca95x4 is an 8-bit I2C IO expander that only exposed register-level access, so its pins couldn't be used through the standard GPIO abstraction. This makes Pca95x4 derive from GpioDriver, consistent with the Pcx857x and Tca955x bindings.

Users can now drive the expander pins through a GpioController:

Pca95x4 pca95x4 = new(i2cDevice);
GpioController controller = new(pca95x4);

controller.OpenPin(0, PinMode.Output);
controller.Write(0, PinValue.High);

controller.OpenPin(1, PinMode.Input);
PinValue value = controller.Read(1);

Changes

• Pca95x4.cs — Derives from GpioDriver; implements the driver members (PinCount, open/close, Set/GetPinMode, IsPinModeSupported, Read, Write) by mapping onto the Configuration/InputPort/OutputPort registers via the existing ReadBit/WriteBit helpers. Event-based members throw NotImplementedException with an explanatory message (mirrors Pcx857x). Disposal is moved into an override of Dispose(bool). The existing register-level public API is unchanged.
• tests/ — New xUnit project covering pin modes, read/write/toggle through GpioController (backed by an in-memory register mock), and a regression check on the register API.
• samples/Program.cs — Adds a UseAsGpioController example.
• README.md — Documents GpioController usage.

Notes

• VirtualGpioController/VirtualGpioPin remap pins from an existing controller and don't supply the hardware driver, so they aren't a substitute here; they can wrap this driver if pin remapping is wanted.
• Pin direction follows the datasheet: a set Configuration bit is an input, a cleared bit is an output.

[pgrawehr]

Looks good to me