Add CMake build system with Linux support and Pong test game

[killerdevildog]

Summary

This PR adds a CMake build system for StarEngine that successfully builds the engine as a static library (libstarengine.a) on Linux. It also includes a fully playable Pong test game that links against the engine, proving the build system works end-to-end.

What was done

CMake Build System

• Added root CMakeLists.txt and src/CMakeLists.txt defining the starengine static library target
• All ~95 source files organized into logical groups (Core, Actions, AI, Components, Graphics, UI, Helpers, Input, Objects, Physics, Scenes, Sound, ThirdParty)
• Platform-conditional compilation for Windows-only files (Console.cpp, Window.cpp) and Android-only files (Resource.cpp, HelpersAndroid.cpp)
• System dependencies resolved via CMake/pkg-config: SDL2, SDL2_mixer, GLEW, OpenGL, Freetype, libpng
• Bundled GLM (header-only) and PugiXML included directly

Linux Portability Fixes

The original codebase targeted Windows (MSVC) and Android (NDK) only. The following changes were needed to compile on Linux/GCC:

• Path separators: Fixed Windows-style backslash \ in #include directives to forward slashes across all source files
• Case-sensitive filenames: Fixed mismatches (SpriteSheet.h → Spritesheet.h, FilePath.h → Filepath.h, etc.)
• Platform guards: The codebase used #ifdef DESKTOP ... #else (assume Android) throughout. Changed to #elif defined(ANDROID) to avoid compiling Android code on Linux
• WGL-specific code: Moved Windows OpenGL extension code (wglext.h, PFNWGLSWAPINTERVALEXTPROC) from #ifdef DESKTOP to #ifdef _WIN32
• Missing types: Added typedef uint8_t byte and tfopen() wrapper for Linux
• MSVC extensions: Replaced unordered_multimap::lower_bound/upper_bound (MSVC-only) with standard equal_range on non-Windows
• Logger: Added Linux implementation using std::cerr output
• GL includes: Fixed include paths for Linux (<GL/glew.h>, <GL/gl.h>)

Test Game: Star Pong

A complete Pong game in test/pong/ that links against libstarengine and uses engine subsystems (Logger, TimeManager):

• Main menu with 1P vs AI and 2P local multiplayer
• AI difficulty selection (Easy / Medium / Hard / Impossible)
• AI with ball trajectory prediction, configurable reaction time and aiming error
• Visual polish: serve countdown, ball trail, paddle glow, screen shake
• Pause and game-over screens
• Custom bitmap font renderer for all UI text

How to build

# Install dependencies (Ubuntu/Debian)
sudo apt install libsdl2-dev libsdl2-mixer-dev libglew-dev libfreetype-dev libpng-dev

# Configure and build
cmake -S . -B build -DSTARENGINE_BUILD_EXAMPLES=ON
cmake --build build -j$(nproc)

# Run the test game
./build/bin/pong

What is NOT in this PR

• Windows build: The existing MSVC/Visual Studio project files are untouched. CMake support for Windows (FindPackage paths, MSVC flags) can be added in a follow-up PR.
• Android build: The NDK/Gradle build integration is not included and would be a separate effort.
• Linux input handling: The InputManager has no SDL-based keyboard/gamepad input for Linux yet — the Pong game handles input directly via SDL. Wiring SDL events into InputManager for Linux would be a future PR.

These platform integrations are straightforward to add incrementally now that the CMake foundation is in place.