Unify minLineClearance across pipeline via ElkLayoutClearance

Add ElkLayoutClearance (thread-static scoped holder) so all 15+
ResolveMinLineClearance call sites in scoring/post-processing use the
same NodeSpacing-aware clearance as the iterative optimizer.

Formula: max(avgNodeSize/2, nodeSpacing * 1.2)
At NodeSpacing=40: max(52.7, 48) = 52.7 (unchanged)
At NodeSpacing=60: max(52.7, 72) = 72 (wider corridors)

The infrastructure is in place. Wider spacing (50+) still needs
routing-level tuning for the different edge convergence patterns
that arise from different node arrangements.

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

src/__Libraries/StellaOps.ElkSharp/ElkEdgeRouterIterative.LocalRepair.ObstacleSkirt.cs

@@ -158,6 +158,12 @@ internal static partial class ElkEdgeRouterIterative
 
     private static double ResolveMinLineClearance(IReadOnlyCollection<ElkPositionedNode> nodes)
     {
+        var overrideClearance = ElkLayoutClearance.Current;
+        if (overrideClearance > 0d)
+        {
+            return overrideClearance;
+        }
+
         var serviceNodes = nodes.Where(node => node.Kind is not "Start" and not "End").ToArray();
         return serviceNodes.Length > 0
             ? Math.Min(serviceNodes.Average(node => node.Width), serviceNodes.Average(node => node.Height)) / 2d