fix(netpacket): Fix undefined behavior with NetPacket buffer writes

[xezon]

• Follow up for refactor(netpacket): Simplify initialization of net packet field types #2288

This change fixes the undefined behavior with NetPacket buffer writes in NetPacket::FillBuffer functions.

TODO

• Test in Network match

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes undefined behavior in all NetPacket::FillBufferWith* serialization functions by replacing placement-new construction directly into raw UnsignedByte* buffers with stack-local construction followed by memcpy.

• Root cause: The NetPacket* structs (defined in NetPacketStructs.h) have non-trivial constructors that initialize field type marker bytes. The target buffers are raw UnsignedByte arrays (NEW UnsignedByte[bufferSize] or m_packet[MAX_PACKET_SIZE]) with no alignment guarantees. Using placement-new on potentially misaligned buffers and then accessing through both raw byte and struct pointers violates strict aliasing rules — all of which constitute undefined behavior.
• Fix: Each of the 20 FillBufferWith* functions now constructs the packet struct as a local stack variable (guaranteeing proper alignment and correct constructor execution), then copies the initialized struct into the buffer using memcpy — the canonical safe serialization pattern in C++.
• Variable data portions (game messages, chat text, file data) that follow the fixed-size struct headers were already using memcpy and remain unchanged, with correct offset calculations referencing sizeof(PacketType).
• The change is purely mechanical and consistent across all affected functions, with no behavioral differences in the serialized output.

Confidence Score: 5/5

• This PR is safe to merge — it replaces undefined behavior with well-defined semantics while producing identical serialized output.
• The change is mechanical and consistent across all 20 FillBufferWith* functions. Each follows the same pattern: construct on stack, populate fields, memcpy to buffer. The struct definitions are pragma-packed to 1-byte alignment ensuring sizeof matches the wire format. Variable data portions already used memcpy and are unchanged. No new logic, no behavioral changes, no edge cases introduced.
• No files require special attention.

Important Files Changed

Filename	Overview
Core/GameEngine/Source/GameNetwork/NetPacket.cpp	Replaces placement-new into raw byte buffers with stack construction + memcpy across all 20 FillBufferWith* functions, eliminating alignment and strict aliasing undefined behavior. All changes are mechanical and consistent.

Flowchart

flowchart TD
    A["FillBufferWithCommand(buffer, ref)"] --> B{Switch on\nNetCommandType}
    B --> C["FillBufferWithGameCommand"]
    B --> D["FillBufferWithAckCommand"]
    B --> E["FillBufferWithFrameCommand"]
    B --> F["...18 other FillBufferWith* functions"]

    subgraph old["Old Pattern (Undefined Behavior)"]
        G["placement-new: NetPacketXxx* p = new (buffer) NetPacketXxx"] --> H["Write fields via p->field = value"]
    end

    subgraph new["New Pattern (Well-Defined)"]
        I["Stack construct: NetPacketXxx packet"] --> J["Write fields via packet.field = value"]
        J --> K["memcpy(buffer, &packet, sizeof(packet))"]
    end

    C --> new
    D --> new
    E --> new
    F --> new

    K --> L["Append variable data via memcpy at offset sizeof(NetPacketXxx)"]

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_{Last reviewed commit: 55e69ea}

[Mauller]

looks good