Add Negative Volume Index (NVI) implementation and tests

- Implemented NVI indicator in Nvi.Quantower.cs with configurable start value and cold value display option.
- Created unit tests for NVI functionality in Nvi.Tests.cs, covering various scenarios including initialization, updates, and edge cases.
- Added validation tests in Nvi.Validation.Tests.cs to ensure NVI matches expected behavior against known implementations.
- Developed comprehensive documentation for NVI in Nvi.md, detailing its historical context, mathematical foundation, and interpretation guide.
- Included error handling for invalid input values and ensured compatibility with volume data.

lib/volume/nvi/Nvi.Quantower.cs

@@ -0,0 +1,53 @@
+using System.Drawing;
+using System.Runtime.CompilerServices;
+using TradingPlatform.BusinessLayer;
+
+namespace QuanTAlib;
+
+[SkipLocalsInit]
+public sealed class NviIndicator : Indicator, IWatchlistIndicator
+{
+    [InputParameter("Start Value", sortIndex: 10, 1, 10000, 1, 0)]
+    public double StartValue { get; set; } = 100;
+
+    [InputParameter("Show cold values", sortIndex: 21)]
+    public bool ShowColdValues { get; set; } = true;
+
+    private Nvi _nvi = null!;
+    private readonly LineSeries _series;
+
+#pragma warning disable S2325 // Instance property required by Quantower indicator interface
+    public int MinHistoryDepths => 2;
+#pragma warning restore S2325
+    int IWatchlistIndicator.MinHistoryDepths => 2;
+
+    public override string ShortName => $"NVI({StartValue})";
+    public override string SourceCodeLink => "https://github.com/mihakralj/QuanTAlib/blob/main/lib/volume/nvi/Nvi.Quantower.cs";
+
+    public NviIndicator()
+    {
+        OnBackGround = true;
+        SeparateWindow = true;
+        Name = "NVI - Negative Volume Index";
+        Description = "Negative Volume Index tracks price changes on days when volume decreases, reflecting smart money activity";
+
+        _series = new LineSeries(name: "NVI", color: Color.DarkCyan, width: 2, style: LineStyle.Solid);
+        AddLineSeries(_series);
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected override void OnInit()
+    {
+        _nvi = new Nvi(StartValue);
+        base.OnInit();
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected override void OnUpdate(UpdateArgs args)
+    {
+        TBar bar = this.GetInputBar(args);
+        TValue result = _nvi.Update(bar, args.IsNewBar());
+
+        _series.SetValue(result.Value, _nvi.IsHot, ShowColdValues);
+    }
+}