feat(tests): add comprehensive tests for LinReg, StdDev, Variance, and Mama indicators; enhance Pwma constructor with null check; improve coverage path mappings in Qodana configuration

lib/statistics/variance/Variance.Tests.cs

@@ -122,4 +122,70 @@ public class VarianceTests
             Assert.Equal(iterativeResults[i], batchResults[i], precision: 7);
         }
     }
+
+    [Fact]
+    public void Update_HandlesConstantValues_ZeroVariance()
+    {
+        var variance = new Variance(5);
+        for (int i = 0; i < 5; i++)
+        {
+            var result = variance.Update(new TValue(DateTime.UtcNow, 10));
+            if (i >= 1) // Variance defined for N >= 2
+            {
+                Assert.Equal(0, result.Value);
+            }
+        }
+    }
+
+    [Fact]
+    public void Update_HandlesNaN()
+    {
+        var variance = new Variance(5);
+        variance.Update(new TValue(DateTime.UtcNow, 1));
+        variance.Update(new TValue(DateTime.UtcNow, 2));
+        variance.Update(new TValue(DateTime.UtcNow, double.NaN));
+        
+        var result = variance.Last.Value;
+        Assert.True(double.IsNaN(result));
+    }
+
+    [Fact]
+    public void Resync_DoesNotDrift()
+    {
+        // Run for > 1000 updates to trigger Resync
+        var variance = new Variance(10);
+        var random = new Random(123);
+        
+        for (int i = 0; i < 1100; i++)
+        {
+            variance.Update(new TValue(DateTime.UtcNow, random.NextDouble() * 100));
+        }
+        
+        Assert.True(double.IsFinite(variance.Last.Value));
+        Assert.True(variance.Last.Value >= 0);
+    }
+
+    [Fact]
+    public void Batch_LargeDataset_Simd()
+    {
+        // Create large dataset to trigger SIMD path (>= 256)
+        int count = 1000;
+        var data = new double[count];
+        for (int i = 0; i < count; i++) data[i] = (double)i;
+
+        var series = new TSeries(new System.Collections.Generic.List<long>(new long[count]), new System.Collections.Generic.List<double>(data));
+        
+        // Batch calculation
+        var batchResult = Variance.Calculate(series, 10);
+        
+        // Verify last value against streaming
+        var variance = new Variance(10);
+        double lastStreaming = 0;
+        foreach (var val in data)
+        {
+            lastStreaming = variance.Update(new TValue(DateTime.UtcNow, val)).Value;
+        }
+
+        Assert.Equal(lastStreaming, batchResult.Last.Value, precision: 10);
+    }
 }