feat(validation): add input validation for Bilateral and Blma constructors; enhance Butter and Mgdi calculations with NaN handling

lib/trends/butter/Butter.Tests.cs

@@ -24,7 +24,7 @@ public class ButterTests
     {
         var source = new double[10];
         var destination = new double[5];
-        Assert.Throws<ArgumentOutOfRangeException>(() => Butter.Calculate(source, destination, 5));
+        Assert.Throws<ArgumentOutOfRangeException>(() => Butter.Calculate(source, destination, 5, double.NaN));
     }
 
     [Fact]
@@ -59,6 +59,14 @@ public class ButterTests
         Assert.Equal(100, result.Value);
     }
 
+    [Fact]
+    public void Initial_NaN_Input_ReturnsNaN()
+    {
+        var butter = new Butter(10);
+        var result = butter.Update(new TValue(DateTime.UtcNow, double.NaN));
+        Assert.True(double.IsNaN(result.Value));
+    }
+
     [Fact]
     public void AllModes_ProduceSameResult()
     {
@@ -74,7 +82,7 @@ public class ButterTests
         var tValues = series.Values.ToArray();
         var spanInput = new ReadOnlySpan<double>(tValues);
         var spanOutput = new double[tValues.Length];
-        Butter.Calculate(spanInput, spanOutput, period);
+        Butter.Calculate(spanInput, spanOutput, period, double.NaN);
         double spanResult = spanOutput[^1];
 
         // 3. Streaming Mode

lib/trends/butter/Butter.cs

@@ -68,6 +68,7 @@ public sealed class Butter : AbstractBase
     {
         _state = new State();
         _p_state = new State();
+        Last = new TValue(0, double.NaN);
     }
 
     public override void Reset()
@@ -97,6 +98,7 @@ public sealed class Butter : AbstractBase
 
         if (double.IsNaN(input.Value) || double.IsInfinity(input.Value))
         {
+            // Return Last (initialized to NaN) if no valid input has been seen yet
             return Last;
         }
 
@@ -126,7 +128,7 @@ public sealed class Butter : AbstractBase
     {
         var result = new TSeries();
         Span<double> output = new double[source.Count];
-        Calculate(source.Values, output, _period);
+        Calculate(source.Values, output, _period, double.NaN);
         
         for (int i = 0; i < source.Count; i++)
         {
@@ -148,7 +150,7 @@ public sealed class Butter : AbstractBase
         return result;
     }
 
-    public static void Calculate(ReadOnlySpan<double> source, Span<double> destination, int period)
+    public static void Calculate(ReadOnlySpan<double> source, Span<double> destination, int period, double initialLast)
     {
         if (period < 2)
         {
@@ -183,7 +185,7 @@ public sealed class Butter : AbstractBase
             double x = source[i];
             if (double.IsNaN(x) || double.IsInfinity(x))
             {
-                destination[i] = i > 0 ? destination[i - 1] : 0;
+                destination[i] = i > 0 ? destination[i - 1] : initialLast;
                 continue;
             }
             double y = i < 2