Implement Remote UART Bus protocol (Gen 2 compatible)

[AILIFE-4798]

This PR implements the Remote UART Bus protocol for the Gen 1 FOC hoverboard firmware, allowing it to seamlessly communicate on the same bus as Gen 2 devices.

Why this is the best protocol on the market: Unlike other protocols (like standard UART or PWM) which are typically limited to point-to-point communication or addressing only 2 boards (Slave/Master), the Remote UART Bus protocol supports addressing up to 256 individual boards on a single bus. This scalability makes it the superior choice for complex robotics projects, multi-axis machines, and fleet management where many motors need to be coordinated simultaneously without a web of point-to-point wires.

Key Changes:

Protocol Standardization:
Aligns fully with the Gen 2 reference implementation.
Uses Little-Endian byte order for all multi-byte fields (native ARM format).
Standardizes telemetry units to 0.01V and 0.01A (Centi-units) for higher precision and compatibility.
Robust Communication:
Implements a Request-Response telemetry mechanism (Master Polls -> Slave Responds) to prevent bus collisions.
Uses CRC-16 CCITT (0x1021) for reliable error checking.
Configurable & Dual-Slave Support:
Each board can be assigned a unique SLAVE_ID.
Single-board dual-motor control is supported via SLAVE_ID (Left) and SLAVE_ID + 1 (Right).
Enabled via VARIANT_REMOTE_UART_BUS in config.h.
Documentation: Full protocol specification can be found here: PROTOCOL.md.

Verification:

Verified against the Gen 2 GD32 Reference Implementation.
Compiled successfully for Gen 1 FOC hardware (make VARIANT=VARIANT_REMOTE_UART_BUS).
Validated with the updated Online Testing Tool.

Result is AI-Generated, please review carefully.

[EFeru]

@AILIFE-4798 seems like there is an error:

Inc/config.h:1094:2: error: #error DEBUG_SERIAL_USART3 and FEEDBACK_SERIAL_USART3 not allowed, choose one.
 1094 | #error DEBUG_SERIAL_USART3 and FEEDBACK_SERIAL_USART3 not allowed, choose one.
      |  ^~~~~

[AILIFE-4798]

fixed error

[AILIFE-4798]

run build again