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

CMakeLists.txt

@@ -158,6 +158,10 @@ set(WOWEE_HEADERS
     include/network/socket.hpp
     include/network/packet.hpp
     include/network/tcp_socket.hpp
+    include/network/world_socket.hpp
+    include/network/net_platform.hpp
+
+    include/platform/process.hpp
 
     include/auth/auth_handler.hpp
     include/auth/auth_opcodes.hpp
@@ -240,6 +244,15 @@ target_link_libraries(wowee PRIVATE
     Threads::Threads
 )
 
+# Platform-specific libraries
+if(WIN32)
+    target_link_libraries(wowee PRIVATE ws2_32)
+    # SDL2main provides WinMain entry point on Windows
+    if(TARGET SDL2::SDL2main)
+        target_link_libraries(wowee PRIVATE SDL2::SDL2main)
+    endif()
+endif()
+
 # Link StormLib if found
 if(STORMLIB_LIBRARY AND STORMLIB_INCLUDE_DIR)
     target_link_libraries(wowee PRIVATE ${STORMLIB_LIBRARY})