Add section z-index constants and complete implementation

Co-authored-by: beto-rodriguez <[email protected]>

Author: Copilot

src/LiveChartsCore/CoreSection.cs

@@ -210,7 +210,7 @@ public override void Invalidate(Chart chart)
 
         if (Fill is not null)
         {
-            Fill.ZIndex = ZIndex ?? -2.5;
+            Fill.ZIndex = ZIndex ?? Paint.SectionFillZIndex;
 
             if (_fillSizedGeometry is null)
             {
@@ -236,7 +236,7 @@ public override void Invalidate(Chart chart)
 
         if (Stroke is not null)
         {
-            Stroke.ZIndex = ZIndex ?? 0;
+            Stroke.ZIndex = ZIndex ?? Paint.SectionStrokeZIndex;
 
             if (_strokeSizedGeometry is null)
             {
@@ -262,7 +262,7 @@ public override void Invalidate(Chart chart)
 
         if (LabelPaint is not null)
         {
-            LabelPaint.ZIndex = ZIndex ?? 0.01;
+            LabelPaint.ZIndex = ZIndex ?? Paint.SectionLabelsZIndex;
 
             var xil = Xi is null ? drawLocation.X : secondaryScale.ToPixels(Xi.Value);
             var yil = Yi is null ? drawLocation.Y : primaryScale.ToPixels(Yi.Value);

src/LiveChartsCore/Painting/Paint.cs

@@ -172,6 +172,22 @@ public Paint(float strokeThickness = 1f, float strokeMiter = 0)
     /// </summary>
     internal const double DrawMarginFrameStrokeZIndex = -0.9;
 
+    // Section-related z-index constants
+    /// <summary>
+    /// Default z-index for section fill.
+    /// </summary>
+    internal const double SectionFillZIndex = -2.5;
+
+    /// <summary>
+    /// Default z-index for section stroke.
+    /// </summary>
+    internal const double SectionStrokeZIndex = 0;
+
+    /// <summary>
+    /// Default z-index for section labels.
+    /// </summary>
+    internal const double SectionLabelsZIndex = 0.01;
+
     #endregion
 
     /// <summary>