Add Windows cross-platform support alongside Linux

Replace POSIX-specific socket and process APIs with portable
abstractions so the project builds on both Windows and Linux.

- Add include/network/net_platform.hpp: Winsock2/POSIX socket
  abstraction (socket types, non-blocking, error handling,
  WSAStartup lifecycle)
- Add include/platform/process.hpp: CreateProcess/fork+exec
  abstraction for spawning ffplay subprocesses
- Update network module (tcp_socket, world_socket) to use
  portable socket helpers instead of raw POSIX calls
- Update audio module (music_manager, footstep_manager,
  activity_sound_manager) to use portable process helpers
  instead of fork/exec/kill/waitpid
- Replace hardcoded /tmp/ paths with std::filesystem::temp_directory_path()
- Link ws2_32 and SDL2main on Windows in CMakeLists.txt

include/audio/music_manager.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "platform/process.hpp"
 #include <string>
 #include <vector>
 
@@ -31,7 +32,7 @@ private:
     pipeline::AssetManager* assetManager = nullptr;
     std::string currentTrack;
     std::string tempFilePath;
-    pid_t playerPid = -1;
+    ProcessHandle playerPid = INVALID_PROCESS;
     bool playing = false;
 
     // Crossfade state