engine almost working, backup before adding gas types.

Core/ThreadLoadTracker.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Threading;
+
+namespace FluidSim
+{
+    /// <summary>
+    /// Tracks the duty cycle of a worker thread using an exponential moving average.
+    /// Thread‑safe: one writer (the sim thread), any reader (UI thread).
+    /// </summary>
+    public class ThreadLoadTracker
+    {
+        private double _loadPercent;   // 0 .. 100, accessed with Volatile.Read/Write
+        private const double Alpha = 0.1;   // smoothing factor (higher = faster response)
+
+        /// <summary>
+        /// Update the load percentage with a new observation.
+        /// </summary>
+        /// <param name="busyMs">Time spent on real work in the last cycle.</param>
+        /// <param name="totalMs">Total time of the last cycle (work + idle). If zero, ignored.</param>
+        public void Record(double busyMs, double totalMs)
+        {
+            if (totalMs <= 0) return;
+            double instantLoad = busyMs / totalMs * 100.0;
+
+            // Exponential moving average
+            double old = Volatile.Read(ref _loadPercent);
+            double newLoad = old + Alpha * (instantLoad - old);
+            Volatile.Write(ref _loadPercent, newLoad);
+        }
+
+        /// <summary>Current smoothed load percentage (0‑100).</summary>
+        public double LoadPercent => Volatile.Read(ref _loadPercent);
+    }
+}