Implement Windows API affinity throttling like working version

- Use SetProcessAffinityMask directly instead of sleep approach
- Limit to first 2 cores (0x3) when user active ~25% usage
- Use all cores (-1) when user inactive
- Add windows.h include for Windows API
- This matches the working implementation approach

src/backend/cpu/CpuWorker.cpp

@@ -20,6 +20,7 @@
 #include <thread>
 #include <mutex>
 #include <algorithm>
+#include <windows.h>
 
 
 #include "backend/cpu/Cpu.h"
@@ -382,11 +383,13 @@ void xmrig::CpuWorker<N>::start()
                 m_count += N;
             }
 
-            // Apply timing-based throttling when user is active
-            // Simple approach: sleep 75% of time to achieve ~25% CPU usage
+            // Apply Windows API affinity throttling when user is active
             if (m_isThrottled) {
-                // Sleep for longer to ensure actual throttling effect
-                std::this_thread::sleep_for(std::chrono::milliseconds(900));
+                // User is active - limit to first 2 cores (~25% on 8-core system)
+                SetProcessAffinityMask(GetCurrentProcess(), 0x3);
+            } else {
+                // User is idle - use all cores
+                SetProcessAffinityMask(GetCurrentProcess(), (DWORD_PTR)-1);
             }
 
             if (m_yield) {