Port FluidSim core and scenarios to C++20

include/FluidSim/ThreadLoadTracker.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <atomic>
+
+namespace fluidsim::port {
+
+class ThreadLoadTracker {
+public:
+    void record(double busy_milliseconds, double budget_milliseconds) {
+        if (budget_milliseconds <= 0.0) return;
+        constexpr double alpha = 0.05;
+        const double instant = busy_milliseconds / budget_milliseconds * 100.0;
+        const double previous = load_percent_.load(std::memory_order_relaxed);
+        load_percent_.store(previous + alpha * (instant - previous), std::memory_order_relaxed);
+        last_busy_ms_.store(busy_milliseconds, std::memory_order_relaxed);
+    }
+    double load_percent() const { return load_percent_.load(std::memory_order_relaxed); }
+    double last_step_microseconds() const { return last_busy_ms_.load(std::memory_order_relaxed) * 1000.0; }
+private:
+    std::atomic<double> load_percent_{};
+    std::atomic<double> last_busy_ms_{};
+};
+
+} // namespace fluidsim::port