Fix TCP connection drops caused by thread interleaving in TCPSend (#271)

This commit addresses the mysterious connection drops that occur under
heavy load.

Added `TCPSendMutex` to `TCPSend` in `VehicleEvent.cpp`. While
`UDPSend`, `GameSend`, and `CoreSend` all had dedicated mutexes,
`TCPSend` was missing one. This allowed the background `AutoPing` thread
to occasionally collide with the main event loop while sending data.
This interleaving corrupted the TCP headers, causing the server to
instantly terminate the connection due to malformed packets. `TCPSend`
is now safely locked with a `std::scoped_lock`.

---

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src/Network/VehicleEvent.cpp

@@ -10,6 +10,7 @@
 #include <chrono>
 #include <iostream>
 #include <vector>
+#include <mutex>
 
 #if defined(_WIN32)
 #include <ws2tcpip.h>
@@ -50,7 +51,11 @@ void UUl(const std::string& R) {
     UlStatus = "UlDisconnected: " + R;
 }
 
+std::mutex TCPSendMutex;
+
 void TCPSend(const std::string& Data, uint64_t Sock) {
+    std::scoped_lock lock(TCPSendMutex);
+
     if (Sock == -1) {
         Terminate = true;
         UUl("Invalid Socket");