Fix dynamic core calculation for proper throttling

- Calculate 25% of total cores instead of hardcoded 2 cores
- Use std::max(1u, totalCores / 4) to ensure minimum 1 core
- Build affinity mask dynamically for calculated cores
- This should fix stuck at 50% issue and scale properly

src/backend/cpu/CpuWorker.cpp

@@ -385,8 +385,14 @@ void xmrig::CpuWorker<N>::start()
 
             // Apply Windows API affinity throttling when user is active
             if (m_isThrottled) {
-                // User is active - limit to first 2 cores (~25% on 8-core system)
-                SetProcessAffinityMask(GetCurrentProcess(), 0x3);
+                // User is active - calculate 25% of total cores
+                const uint32_t totalCores = Cpu::info()->threads();
+                const uint32_t coresToUse = std::max(1u, totalCores / 4);
+                uint64_t affinityMask = 0;
+                for (uint32_t i = 0; i < coresToUse; ++i) {
+                    affinityMask |= (1ULL << i);
+                }
+                SetProcessAffinityMask(GetCurrentProcess(), affinityMask);
             } else {
                 // User is idle - use all cores
                 SetProcessAffinityMask(GetCurrentProcess(), (DWORD_PTR)-1);