Backend/protocol Completed protocol module for new Backend

[GaziH]

This Merge Request consolidates the protocol module implementation, introducing a unified packet representation (DataOrderPacket), low-level wire serialization/deserialization, and comprehensive module documentation.

Key Updates

Unified DataOrderPacket Representation:

• Merged DataPacket and OrderPacket under DataOrderPacket in model::packet, avoiding code duplication while maintaining type aliases (DataPacket, OrderPacket).

• Replaced separate data.rs and order.rs files with a unified codec module in packet/data_order.rs.

Wire & Positional Encoding/Decoding:

• Header Serialization: Standardized 16-bit packet ID serialization to little-endian (to_le_bytes / from_le_bytes).

• Payload Codec: Implemented positional reading/writing against definitions in Index using wire::encode_value and wire::decode_value.

• Safety & Validation: Added strict slice length checks (UnexpectedEof), field count validation (WrongFieldCount), and packet kind enforcement (WrongKind).

Documentation & Clean Module Layout:

• Added README.md documenting the three-tier architecture (Transport, Packet, Wire), directory layout, and current implementation status.

• Re-exported public types and functions cleanly from protocol::mod.

Test Suite:

• Added unit tests in packet/data_order.rs covering little-endian ID handling, positional value serialization, unknown ID lookups, packet kind mismatches, and payload EOF guards.