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/activity_sound_manager.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "audio/footstep_manager.hpp"
+#include "platform/process.hpp"
 #include <array>
 #include <chrono>
 #include <cstdint>
@@ -51,10 +52,10 @@ private:
 
     bool swimmingActive = false;
     bool swimMoving = false;
-    pid_t swimLoopPid = -1;
-    pid_t oneShotPid = -1;
-    std::string loopTempPath = "/tmp/wowee_swim_loop.wav";
-    std::string oneShotTempPath = "/tmp/wowee_activity.wav";
+    ProcessHandle swimLoopPid = INVALID_PROCESS;
+    ProcessHandle oneShotPid = INVALID_PROCESS;
+    std::string loopTempPath = platform::getTempFilePath("wowee_swim_loop.wav");
+    std::string oneShotTempPath = platform::getTempFilePath("wowee_activity.wav");
     std::mt19937 rng;
 
     std::chrono::steady_clock::time_point lastJumpAt{};

include/audio/footstep_manager.hpp

@@ -1,10 +1,10 @@
 #pragma once
 
+#include "platform/process.hpp"
 #include <cstdint>
 #include <random>
 #include <string>
 #include <vector>
-#include <sys/types.h>
 #include <chrono>
 
 namespace wowee {
@@ -56,8 +56,8 @@ private:
     SurfaceSamples surfaces[7];
     size_t sampleCount = 0;
 
-    std::string tempFilePath = "/tmp/wowee_footstep.wav";
-    pid_t playerPid = -1;
+    std::string tempFilePath = platform::getTempFilePath("wowee_footstep.wav");
+    ProcessHandle playerPid = INVALID_PROCESS;
     std::chrono::steady_clock::time_point lastPlayTime = std::chrono::steady_clock::time_point{};
 
     std::mt19937 rng;

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