Add Y-axis gutter expansion for routing-aware node placement

The Y-axis counterpart to ExpandVerticalCorridorGutters: after edges
are routed, detects horizontal segments with under-node or alongside
violations, then inserts horizontal gutters by shifting all nodes
below the violation point downward. Re-routes with expanded corridors.

This is the architectural fix for the placement-routing disconnect:
instead of patching edge paths after routing (corridor reroute,
push-down, spread), the gutter expansion creates adequate routing
corridors in the node placement so edges route cleanly.

Runs after X-gutters and before compact passes, up to 2 iterations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/__Libraries/StellaOps.ElkSharp/ElkEdgeChannelGutters.HorizontalRouting.cs

@@ -0,0 +1,134 @@
+namespace StellaOps.ElkSharp;
+
+/// <summary>
+/// Y-axis counterpart to <see cref="ElkEdgeChannelGutters.ExpandVerticalCorridorGutters"/>.
+/// After edges are routed, detects horizontal segments that pass too close to
+/// (or flush with) non-source/target nodes. Inserts horizontal "gutters" by
+/// shifting all nodes below the violation Y downward, creating routing space.
+///
+/// This is the architectural fix for under-node, alongside, and shared-lane
+/// violations: instead of patching edge paths after routing, create adequate
+/// routing corridors in the node placement so edges route cleanly on the first pass.
+/// </summary>
+internal static class ElkEdgeHorizontalRoutingGutters
+{
+    internal static bool ExpandHorizontalRoutingGutters(
+        Dictionary<string, ElkPositionedNode> positionedNodes,
+        IReadOnlyCollection<ElkRoutedEdge> routedEdges,
+        IReadOnlyDictionary<string, ElkNode> nodesById,
+        double nodeSpacing,
+        ElkLayoutDirection direction)
+    {
+        if (direction != ElkLayoutDirection.LeftToRight)
+        {
+            return false;
+        }
+
+        var nodes = positionedNodes.Values.ToArray();
+        if (nodes.Length == 0)
+        {
+            return false;
+        }
+
+        var serviceNodes = nodes.Where(n => n.Kind is not "Start" and not "End").ToArray();
+        var minClearance = serviceNodes.Length > 0
+            ? Math.Min(serviceNodes.Average(n => n.Width), serviceNodes.Average(n => n.Height)) / 2d
+            : 50d;
+
+        // Scan routed edges for horizontal segments with under-node or alongside violations.
+        var gutterRequirements = new Dictionary<double, double>(); // gutterY → requiredShift
+
+        foreach (var edge in routedEdges)
+        {
+            foreach (var segment in ElkEdgeRoutingGeometry.FlattenSegments(edge))
+            {
+                if (Math.Abs(segment.Start.Y - segment.End.Y) > 2d)
+                {
+                    continue; // not horizontal
+                }
+
+                var laneY = segment.Start.Y;
+                var minX = Math.Min(segment.Start.X, segment.End.X);
+                var maxX = Math.Max(segment.Start.X, segment.End.X);
+
+                foreach (var node in nodes)
+                {
+                    if (string.Equals(node.Id, edge.SourceNodeId, StringComparison.Ordinal)
+                        || string.Equals(node.Id, edge.TargetNodeId, StringComparison.Ordinal))
+                    {
+                        continue;
+                    }
+
+                    // Check X overlap
+                    if (maxX <= node.X + 0.5d || minX >= node.X + node.Width - 0.5d)
+                    {
+                        continue;
+                    }
+
+                    var nodeBottom = node.Y + node.Height;
+
+                    // Under-node: lane is below node bottom within clearance
+                    var gapBelow = laneY - nodeBottom;
+                    if (gapBelow > -4d && gapBelow < minClearance)
+                    {
+                        var needed = minClearance - gapBelow + 4d;
+                        var gutterY = nodeBottom;
+                        if (!gutterRequirements.TryGetValue(gutterY, out var existing) || needed > existing)
+                        {
+                            gutterRequirements[gutterY] = needed;
+                        }
+                    }
+
+                    // Above-node: lane is above node top within clearance
+                    var gapAbove = node.Y - laneY;
+                    if (gapAbove > -4d && gapAbove < minClearance)
+                    {
+                        var needed = minClearance - gapAbove + 4d;
+                        var gutterY = laneY - needed;
+                        if (!gutterRequirements.TryGetValue(gutterY, out var existing) || needed > existing)
+                        {
+                            gutterRequirements[gutterY] = needed;
+                        }
+                    }
+                }
+            }
+        }
+
+        if (gutterRequirements.Count == 0)
+        {
+            return false;
+        }
+
+        // Sort gutters from bottom to top (process bottom-most first to avoid
+        // cascading shifts that change gutter positions).
+        var orderedGutters = gutterRequirements
+            .OrderByDescending(g => g.Key)
+            .ToArray();
+
+        // Apply shifts: for each gutter, shift all nodes whose center Y is below
+        // the gutter Y downward by the required amount.
+        foreach (var (gutterY, requiredShift) in orderedGutters)
+        {
+            foreach (var nodeId in positionedNodes.Keys.ToArray())
+            {
+                var node = positionedNodes[nodeId];
+                var nodeCenterY = node.Y + (node.Height / 2d);
+
+                if (nodeCenterY <= gutterY)
+                {
+                    continue; // node is above the gutter — don't shift
+                }
+
+                if (!nodesById.TryGetValue(nodeId, out var nodeSpec))
+                {
+                    continue;
+                }
+
+                positionedNodes[nodeId] = ElkLayoutHelpers.CreatePositionedNode(
+                    nodeSpec, node.X, node.Y + requiredShift, direction);
+            }
+        }
+
+        return true;
+    }
+}