fix: snake gradient solid bands under circles and proportional title sizing

- Gradient stops now use paired offsets so each circle gets a solid color band matching the circle width, with smooth gradient transitions only in the gaps between circles.

- Snake title font size is now proportional to the circle labels instead of using the small theme default. Title Y position includes proper padding from the top.

- Updated E2E snapshot baselines for both presentation and dracula fixtures.

Author: jongalloway

src/DiagramForge/Layout/DefaultLayoutEngine.Snake.cs

@@ -20,7 +20,13 @@ private static void LayoutSnakeDiagram(
             return;
 
         double fontSize = theme.FontSize;
-        double titleOffset = !string.IsNullOrWhiteSpace(diagram.Title) ? theme.TitleFontSize * 1.4 + 12 : 0;
+
+        // Snake title uses a font size proportional to the circles (set below
+        // once snakeFontSize is known).  Reserve a placeholder offset here;
+        // it is updated after sizing.
+        bool hasTitle = !string.IsNullOrWhiteSpace(diagram.Title);
+        double snakeTitleFontSize = 0;
+        double titleOffset = 0;
 
         // ── Wrap all multi-word labels to two lines for larger text ───────────
         foreach (var node in orderedNodes)
@@ -88,6 +94,10 @@ private static void LayoutSnakeDiagram(
             node.Metadata["icon:y"] = iconY;
         }
 
+        // Snake title size — match the circle labels so it reads well
+        snakeTitleFontSize = snakeFontSize;
+        titleOffset = hasTitle ? snakeTitleFontSize * 1.4 + pad : 0;
+
         // Description text blocks — proportional to circle size for readability
         double descFontSize = snakeFontSize * 0.85;
         double descMaxWidth = circleDiameter * 2.0;
@@ -209,6 +219,11 @@ private static void LayoutSnakeDiagram(
         diagram.Metadata["snake:segmentStops"] = segmentStops;
         diagram.Metadata["snake:nodeCount"] = n;
         diagram.Metadata["snake:descFontSize"] = descFontSize;
+        if (hasTitle)
+        {
+            diagram.Metadata["titleFontSize"] = snakeTitleFontSize;
+            diagram.Metadata["titleY"] = pad + snakeTitleFontSize;
+        }
 
         // ── Store description text positions ──────────────────────────────────
         for (int i = 0; i < orderedNodes.Count; i++)

src/DiagramForge/Rendering/SvgRenderer.cs

@@ -36,10 +36,16 @@ public string Render(Diagram diagram, Theme theme)
             sb.AppendLine($"""  <rect width="{SvgRenderSupport.F(width)}" height="{SvgRenderSupport.F(height)}" fill="{SvgRenderSupport.Escape(theme.BackgroundColor)}" rx="{SvgRenderSupport.F(theme.BorderRadius)}" ry="{SvgRenderSupport.F(theme.BorderRadius)}"/>""");
         }
 
-        // Title
+        // Title — diagram-type layouts may override font size / position via metadata
         if (!string.IsNullOrWhiteSpace(diagram.Title))
         {
-            sb.AppendLine($"""  <text x="{SvgRenderSupport.F(width / 2)}" y="{SvgRenderSupport.F(theme.DiagramPadding - 4)}" text-anchor="middle" font-family="{SvgRenderSupport.Escape(theme.FontFamily)}" font-size="{SvgRenderSupport.F(theme.TitleFontSize)}" font-weight="bold" fill="{SvgRenderSupport.Escape(theme.TitleTextColor)}">{SvgRenderSupport.Escape(diagram.Title)}</text>""");
+            double titleFontSize = diagram.Metadata.TryGetValue("titleFontSize", out var tfsObj)
+                ? Convert.ToDouble(tfsObj, System.Globalization.CultureInfo.InvariantCulture)
+                : theme.TitleFontSize;
+            double titleY = diagram.Metadata.TryGetValue("titleY", out var tyObj)
+                ? Convert.ToDouble(tyObj, System.Globalization.CultureInfo.InvariantCulture)
+                : theme.DiagramPadding - 4;
+            sb.AppendLine($"""  <text x="{SvgRenderSupport.F(width / 2)}" y="{SvgRenderSupport.F(titleY)}" text-anchor="middle" font-family="{SvgRenderSupport.Escape(theme.FontFamily)}" font-size="{SvgRenderSupport.F(titleFontSize)}" font-weight="bold" fill="{SvgRenderSupport.Escape(theme.TitleTextColor)}">{SvgRenderSupport.Escape(diagram.Title)}</text>""");
         }
 
         // Groups (render behind nodes). Parents render first so nested child groups