[CORE] Add and configure all third-party dependencies

[mrgreenapple24]

Goal

Vendor all required third-party libraries into external/ and integrate them into the build.

Dependencies to Add

• GLFW (windowing)
• GLAD (OpenGL loader — generate for OpenGL 4.6 Core)
• GLM (math)
• Dear ImGui — docking branch
• spdlog
• stb_image
• EnTT
• Sol2 + Lua 5.4 (or LuaJIT)
• yaml-cpp
• (Optional) zstd for future PAK compression

Tasks

• Add each as a git submodule.
• Write Premake/CMake entries for each so they compile as part of the build
• Create an umbrella external.h or per-system include wrappers
• Verify all dependencies compile without warnings on target platform

Acceptance Criteria

All libraries link correctly into the Engine project with no errors.