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/network/net_platform.hpp

@@ -0,0 +1,122 @@
+#pragma once
+
+// Cross-platform socket abstractions for Windows (Winsock2) and POSIX.
+
+#ifdef _WIN32
+  #ifndef WIN32_LEAN_AND_MEAN
+    #define WIN32_LEAN_AND_MEAN
+  #endif
+  #include <winsock2.h>
+  #include <ws2tcpip.h>
+  #pragma comment(lib, "ws2_32.lib")
+
+  using socket_t  = SOCKET;
+  using ssize_t   = int;             // recv/send return int on Windows
+
+  inline constexpr socket_t INVALID_SOCK = INVALID_SOCKET;
+
+#else
+  #include <sys/socket.h>
+  #include <netinet/in.h>
+  #include <arpa/inet.h>
+  #include <netdb.h>
+  #include <unistd.h>
+  #include <fcntl.h>
+  #include <cerrno>
+
+  using socket_t = int;
+
+  inline constexpr socket_t INVALID_SOCK = -1;
+
+#endif
+
+#include <cstring>
+
+namespace wowee {
+namespace net {
+
+// ---- Winsock lifecycle (no-op on Linux) ----
+
+#ifdef _WIN32
+struct WinsockInit {
+    WinsockInit() {
+        WSADATA wsa;
+        WSAStartup(MAKEWORD(2, 2), &wsa);
+    }
+    ~WinsockInit() { WSACleanup(); }
+};
+// Call once at program start (e.g. as a static in Application).
+inline void ensureInit() {
+    static WinsockInit instance;
+}
+#else
+inline void ensureInit() {}
+#endif
+
+// ---- Portable helpers ----
+
+inline void closeSocket(socket_t s) {
+#ifdef _WIN32
+    closesocket(s);
+#else
+    close(s);
+#endif
+}
+
+inline bool setNonBlocking(socket_t s) {
+#ifdef _WIN32
+    u_long mode = 1;
+    return ioctlsocket(s, FIONBIO, &mode) == 0;
+#else
+    int flags = fcntl(s, F_GETFL, 0);
+    return fcntl(s, F_SETFL, flags | O_NONBLOCK) != -1;
+#endif
+}
+
+inline int lastError() {
+#ifdef _WIN32
+    return WSAGetLastError();
+#else
+    return errno;
+#endif
+}
+
+inline bool isWouldBlock(int err) {
+#ifdef _WIN32
+    return err == WSAEWOULDBLOCK;
+#else
+    return err == EAGAIN || err == EWOULDBLOCK;
+#endif
+}
+
+inline bool isInProgress(int err) {
+#ifdef _WIN32
+    return err == WSAEWOULDBLOCK || err == WSAEALREADY;
+#else
+    return err == EINPROGRESS;
+#endif
+}
+
+inline const char* errorString(int err) {
+#ifdef _WIN32
+    // Simple thread-local buffer for FormatMessage
+    thread_local char buf[256];
+    FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
+                   nullptr, err, 0, buf, sizeof(buf), nullptr);
+    return buf;
+#else
+    return strerror(err);
+#endif
+}
+
+// Portable send — Windows recv/send take char*, not void*.
+inline ssize_t portableSend(socket_t s, const uint8_t* data, size_t len) {
+    return ::send(s, reinterpret_cast<const char*>(data), static_cast<int>(len), 0);
+}
+
+inline ssize_t portableRecv(socket_t s, uint8_t* buf, size_t len) {
+    return ::recv(s, reinterpret_cast<char*>(buf), static_cast<int>(len), 0);
+}
+
+} // namespace net
+} // namespace wowee

include/network/tcp_socket.hpp

@@ -1,7 +1,7 @@
 #pragma once
 
 #include "network/socket.hpp"
-#include <sys/socket.h>
+#include "network/net_platform.hpp"
 
 namespace wowee {
 namespace network {
@@ -22,7 +22,7 @@ private:
     void tryParsePackets();
     size_t getExpectedPacketSize(uint8_t opcode);
 
-    int sockfd = -1;
+    socket_t sockfd = INVALID_SOCK;
     bool connected = false;
     std::vector<uint8_t> receiveBuffer;
 };

include/network/world_socket.hpp

@@ -2,6 +2,7 @@
 
 #include "network/socket.hpp"
 #include "network/packet.hpp"
+#include "network/net_platform.hpp"
 #include "auth/rc4.hpp"
 #include <functional>
 #include <vector>
@@ -73,7 +74,7 @@ private:
      */
     void tryParsePackets();
 
-    int sockfd = -1;
+    socket_t sockfd = INVALID_SOCK;
     bool connected = false;
     bool encryptionEnabled = false;