Fix chevron diagram shape: inward V-notch with overlap layout

Non-first chevron nodes were incorrectly protruding leftward (<> shape).
Fix uses an inward V-notch polygon and overlapping layout so each node's
right tip aligns exactly with the next node's notch.

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Author: Copilot

src/DiagramForge/Layout/DefaultLayoutEngine.Chevron.cs

@@ -37,8 +37,9 @@ private static void LayoutChevronDiagram(
         {
             var node = orderedNodes[index];
 
-            // Zero gap: bounding boxes abut so the tip of stage N meets the notch of stage N+1.
-            node.X = pad + index * nodeW;
+            // Overlap layout: each stage overlaps the previous by tipDepth so the right tip of
+            // stage N aligns exactly with the inward notch of stage N+1.
+            node.X = pad + index * (nodeW - tipDepth);
             node.Y = pad + titleOffset;
             node.Width = nodeW;
             node.Height = nodeH;

src/DiagramForge/Rendering/SvgNodeWriter.cs

@@ -241,10 +241,12 @@ private static void AppendChevronNode(StringBuilder sb, Node node, string fill,
         double midY = h / 2;
 
         // First chevron: flat left edge, pointed right – 5 points (pentagon).
-        // Subsequent chevrons: notched left edge matching previous arrow point, pointed right – 6 points (hexagon).
+        // Subsequent chevrons: inward V-notch on left matching previous arrow tip, pointed right – 6 points (hexagon).
+        // The notch vertex at (tipDepth, midY) aligns with the tip of the preceding stage because
+        // the layout overlaps bounding boxes by tipDepth.
         string points = index == 0
             ? $"0,0 {SvgRenderSupport.F(w - tipDepth)},0 {SvgRenderSupport.F(w)},{SvgRenderSupport.F(midY)} {SvgRenderSupport.F(w - tipDepth)},{SvgRenderSupport.F(h)} 0,{SvgRenderSupport.F(h)}"
-            : $"0,{SvgRenderSupport.F(midY)} {SvgRenderSupport.F(tipDepth)},0 {SvgRenderSupport.F(w - tipDepth)},0 {SvgRenderSupport.F(w)},{SvgRenderSupport.F(midY)} {SvgRenderSupport.F(w - tipDepth)},{SvgRenderSupport.F(h)} {SvgRenderSupport.F(tipDepth)},{SvgRenderSupport.F(h)}";
+            : $"0,0 {SvgRenderSupport.F(w - tipDepth)},0 {SvgRenderSupport.F(w)},{SvgRenderSupport.F(midY)} {SvgRenderSupport.F(w - tipDepth)},{SvgRenderSupport.F(h)} 0,{SvgRenderSupport.F(h)} {SvgRenderSupport.F(tipDepth)},{SvgRenderSupport.F(midY)}";
 
         sb.AppendLine($"""    <polygon points="{points}" fill="{fill}" stroke="{stroke}" stroke-width="{SvgRenderSupport.F(theme.StrokeWidth)}"{fillOpacityAttribute}{shadowAttribute}/>""");
     }

tests/DiagramForge.E2ETests/Fixtures/conceptual-chevrons.expected.svg

@@ -1362,8 +1362,11 @@ public void Layout_Chevrons_AdjacentNodesAreImmediatelyAbutted()
         var first = diagram.Nodes["node_0"];
         var second = diagram.Nodes["node_1"];
 
-        // Zero gap: the tip of stage 0 meets the notch of stage 1 at their shared boundary.
-        Assert.Equal(first.X + first.Width, second.X, precision: 6);
+        // Overlap layout: the right tip of stage 0 aligns with the inward notch of stage 1.
+        // second.X is inset by tipDepth so the tip (first.X + first.Width) coincides with
+        // the notch vertex (second.X + tipDepth).
+        double tipDepth = (double)second.Metadata["conceptual:chevronTipDepth"];
+        Assert.Equal(first.X + first.Width, second.X + tipDepth, precision: 6);
     }
 
     [Fact]