Manage header include guards

I need the header to be safe to include multiple times in large builds. Add an include guard to game_engine/math/vec3.h so the file can be included from renderer, physics and tools without accidental redefinition. Use the project convention of uppercase path-based macro names and place the guard around everything in the file; leave the existing content unchanged otherwise.

Before I commit vec3.h to the repo, make approving it trivial for reviewers. Put the include guard macro name at the top so reviewers can scan it, keep the guard consistent with other headers (GAME_ENGINE_MATH_VEC3_H), and ensure there is exactly one trailing newline. Flag any existing #pragma once and explain why we prefer the macro style here.

I just got multiple-definition link errors when combining renderer and physics, both include game_engine/math/vec3.h. I think the header lacks a proper include guard or has inconsistent macros. I cannot check every include path and I worry changing the header could break generated code. What's the most likely cause and the safest fix to restore a clean build now?

Across several projects we keep getting duplicate-definition or redefinition errors after refactors. We patch headers ad hoc with different guard styles and rely on developers remembering to add guards. What one habit or small rule change will stop these recurring rebuild-and-fix cycles and make code reviews catch the problem earlier?