fix(network): Move TransportMessageHeader to before it is used in NetworkDefs.h

[Mauller]

This PR fixes a compilation error due to the struct TransportMessageHeader being defined after code attempts to obtain it's size.

Moved TransportMessageHeader before the block of static varaibles where it is first used.

[greptile-apps]

Greptile Summary

Fixed compilation error by moving the TransportMessageHeader struct definition before its usage in sizeof() on line 77.

• Relocated struct definition from after line 97 to after line 49, before the static constant declarations
• The MAX_PACKET_SIZE constant calculation requires the complete struct definition to determine its size
• No functional changes, only reordering of declarations to satisfy C++ forward declaration requirements

Confidence Score: 5/5

• This PR is safe to merge with no risk
• Simple struct relocation to fix forward declaration order - no logic changes, no functional impact beyond fixing compilation
• No files require special attention

Important Files Changed

Filename	Overview
Core/GameEngine/Include/GameNetwork/NetworkDefs.h	Moved TransportMessageHeader struct definition before sizeof() usage to fix compilation error

_{Last reviewed commit: f31ceca}