refactor: optimize string operations and improve code clarity

- Avoid redundant substr() calls in packet parsing hot-path (TServer.cpp)
  The previous code called substr(3) twice per packet, creating
  unnecessary temporary strings. Now stores the result once.

- Replace .size() == 0 with .empty() for idiomatic C++
  (TConsole.cpp, TLuaEngine.cpp)

src/TLuaEngine.cpp

@@ -131,7 +131,7 @@ void TLuaEngine::operator()() {
                 }
             }
         }
-        if (mLuaStates.size() == 0) {
+        if (mLuaStates.empty()) {
             beammp_trace("No Lua states, event loop running extremely sparsely");
             Application::SleepSafeSeconds(10);
         } else {