refactor: move Decay/Edecay from trends_IIR to numerics; update filter signatures

- Move lib/trends_IIR/decay/ → lib/numerics/decay/
- Move lib/trends_IIR/edecay/ → lib/numerics/edecay/
- Update Category in Decay.md/Edecay.md from Trends (IIR) to Numerics
- Add DECAY/EDECAY entries to lib/numerics/_index.md and docs/indicators.md
- Update filter signature .md files and .svg assets
- Update trends_IIR signature docs (htit, mama, holt, etc.)
- All 163 tests passing, 0 warnings, 0 errors

lib/numerics/decay/Decay.Quantower.cs

@@ -0,0 +1,60 @@
+using System.Drawing;
+using System.Runtime.CompilerServices;
+using TradingPlatform.BusinessLayer;
+
+namespace QuanTAlib;
+
+/// <summary>
+/// DECAY (Linear Decay) Quantower indicator.
+/// Tracks peaks and decays linearly at a rate of 1/period per bar.
+/// Formula: output = max(input, prev_output - 1/period)
+/// </summary>
+[SkipLocalsInit]
+public class DecayIndicator : Indicator, IWatchlistIndicator
+{
+    [InputParameter("Period", sortIndex: 1, 1, 1000, 1, 0)]
+    public int Period { get; set; } = 5;
+
+    [IndicatorExtensions.DataSourceInput]
+    public SourceType Source { get; set; } = SourceType.Close;
+
+    [InputParameter("Show cold values", sortIndex: 21)]
+    public bool ShowColdValues { get; set; } = true;
+
+    private Decay _decay = null!;
+    protected LineSeries Series;
+    protected string SourceName = null!;
+    private Func<IHistoryItem, double> _priceSelector = null!;
+
+    public static int MinHistoryDepths => 0;
+    int IWatchlistIndicator.MinHistoryDepths => MinHistoryDepths;
+
+    public override string ShortName => $"DECAY {Period}:{SourceName}";
+
+    public DecayIndicator()
+    {
+        OnBackGround = true;
+        SeparateWindow = false;
+        SourceName = Source.ToString();
+        Name = "DECAY - Linear Decay";
+        Description = "Linear Decay: output = max(input, prev_output - 1/period)";
+        Series = new LineSeries(name: $"DECAY {Period}", color: IndicatorExtensions.Averages, width: 2, style: LineStyle.Solid);
+        AddLineSeries(Series);
+    }
+
+    protected override void OnInit()
+    {
+        _decay = new Decay(Period);
+        SourceName = Source.ToString();
+        _priceSelector = Source.GetPriceSelector();
+        base.OnInit();
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected override void OnUpdate(UpdateArgs args)
+    {
+        var item = HistoricalData[Count - 1, SeekOriginHistory.Begin];
+        TValue result = _decay.Update(new TValue(item.TimeLeft.Ticks, _priceSelector(item)), isNew: args.IsNewBar());
+        Series.SetValue(result.Value, _decay.IsHot, ShowColdValues);
+    }
+}