Add serial parity option (M575 F) for device/Modbus mode

Aux serial ports in device (Modbus RTU) mode previously always opened as
8N1. Some Modbus devices require parity - e.g. OnRobot grippers need 8E1 -
so add an optional parity setting (#1196).

- AuxDevice: store a SerialParity (none/even/odd) and apply it in SetMode
  by selecting Mode_8N1/8E1/8O1 when calling AsyncSerial::begin(baudRate,
  config).
- M575: in device mode, parse the F parameter (F0 none/8N1, F1 even/8E1,
  F2 odd/8O1). The parameter is applied on every device-mode M575, so
  omitting F restores the previous behaviour (8N1) instead of retaining a
  previously set parity.

The core (CoreN2G AsyncSerial) already defines the Mode_8E1/8O1 framing,
so no core change is needed. Default stays 8N1, so PanelDue/raw modes are
unaffected.

Enables e.g.: M575 P1 S7 B115200 F1  (Modbus RTU, 115200, 8E1).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: filippo-bosi

src/Comms/AuxDevice.cpp

@@ -42,7 +42,10 @@ void AuxDevice::SetMode(AuxMode p_mode) noexcept
 #if SUPPORT_MODBUS_RTU
 			uart->SetOnTxEndedCallback((p_mode == AuxMode::device) ? GlobalTxEndedCallback : nullptr, CallbackParameter(this));
 #endif
-			uart->begin(baudRate);
+			AsyncSerial::UARTModes config = AsyncSerial::Mode_8N1;
+			if (serialParity == SerialParity::even) { config = AsyncSerial::Mode_8E1; }
+			else if (serialParity == SerialParity::odd) { config = AsyncSerial::Mode_8O1; }
+			uart->begin(baudRate, config);
 			mode = p_mode;
 		}
 	}

src/Comms/AuxDevice.h

@@ -31,11 +31,14 @@ class AsyncSerial;
 class AuxDevice
 {
 public:
+	enum class SerialParity : uint8_t { none = 0, even, odd };
+
 	AuxDevice() noexcept;
 
 	void Init(AsyncSerial *p_uart, uint32_t p_baudRate) noexcept;
 	bool IsEnabledForGCodeIo() const noexcept { return mode == AuxMode::raw || mode == AuxMode::panelDue; }
 	void SetMode(AuxMode p_mode) noexcept;
+	void SetSerialParity(SerialParity p_parity) noexcept { serialParity = p_parity; }	// applied on next SetMode; affects device/Modbus mode only
 	void SetBaudRate(uint32_t p_baudRate) noexcept { baudRate = p_baudRate; }			// must call SetMode after calling this to actually change the baud rate
 	void Disable() noexcept;
 	AuxMode GetMode() const noexcept { return mode; }
@@ -91,6 +94,8 @@ class AuxDevice
 	uint32_t baudRate;
 	AuxMode mode = AuxMode::disabled;		// whether disabled, raw, PanelDue mode or Modbus RTU mode
 
+	SerialParity serialParity = SerialParity::none;		// device/Modbus serial parity: none=8N1, even=8E1, odd=8O1
+
 #if SUPPORT_MODBUS_RTU
 	IoPort txNotRx;							// port used to switch the RS485 port between transmit and receive
 	uint8_t *_ecv_array receivedData;

src/Platform/Platform.cpp

@@ -2303,6 +2303,14 @@ GCodeResult Platform::HandleM575(GCodeBuffer& gb, const StringRef& reply) THROWS
 				}
 			}
 #  endif
+			// Serial parity for device/Modbus mode. Always applied so that omitting F restores
+			// the pre-existing behaviour (no parity, 8N1) rather than keeping a previous setting.
+			switch (gb.Seen('F') ? gb.GetLimitedUIValue('F', 3) : 0)
+			{
+			case 1:		dev.SetSerialParity(AuxDevice::SerialParity::even); break;
+			case 2:		dev.SetSerialParity(AuxDevice::SerialParity::odd); break;
+			default:	dev.SetSerialParity(AuxDevice::SerialParity::none); break;
+			}
 # endif
 		}