Add Ehlers Hilbert Transform Instantaneous Trend (HTIT) implementation and tests

- Implemented the HTIT indicator in Htit.cs, utilizing the Hilbert Transform for trend analysis.
- Added unit tests for HTIT validation against TA-Lib, Skender, and Ooples implementations in Htit.Validation.Tests.cs.
- Created documentation for HTIT in Htit.md, detailing its core concepts, formula, parameters, usage, and interpretation.

lib/core/ringbuffer/RingBuffer.Tests.cs

@@ -437,19 +437,19 @@ public class RingBufferTests
     }
 
     [Fact]
-    public void Newest_EmptyBuffer_ReturnsZero()
+    public void Newest_EmptyBuffer_ReturnsNaN()
     {
         var buffer = new RingBuffer(5);
 
-        Assert.Equal(0, buffer.Newest);
+        Assert.True(double.IsNaN(buffer.Newest));
     }
 
     [Fact]
-    public void Oldest_EmptyBuffer_ReturnsZero()
+    public void Oldest_EmptyBuffer_ReturnsNaN()
     {
         var buffer = new RingBuffer(5);
 
-        Assert.Equal(0, buffer.Oldest);
+        Assert.True(double.IsNaN(buffer.Oldest));
     }
 
     [Fact]

lib/core/ringbuffer/RingBuffer.cs

@@ -110,13 +110,14 @@ public sealed class RingBuffer : IEnumerable<double>
 
     /// <summary>
     /// Gets the newest (most recently added) value.
+    /// Returns double.NaN if buffer is empty.
     /// </summary>
     public double Newest
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         get
         {
-            if (_count == 0) return 0;
+            if (_count == 0) return double.NaN;
             int idx = (_head - 1 + _capacity) % _capacity;
             return _buffer[idx];
         }
@@ -124,13 +125,14 @@ public sealed class RingBuffer : IEnumerable<double>
 
     /// <summary>
     /// Gets the oldest value in the buffer.
+    /// Returns double.NaN if buffer is empty.
     /// </summary>
     public double Oldest
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         get
         {
-            if (_count == 0) return 0;
+            if (_count == 0) return double.NaN;
             int start = _count == _capacity ? _head : 0;
             return _buffer[start];
         }