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/ElkEdgeChannelGutters.HorizontalRouting.cs

@@ -34,6 +34,12 @@ internal static class ElkEdgeHorizontalRoutingGutters
         var minClearance = serviceNodes.Length > 0
             ? Math.Min(serviceNodes.Average(n => n.Width), serviceNodes.Average(n => n.Height)) / 2d
             : 50d;
+        // Use layout-wide clearance if available (scales with NodeSpacing).
+        var overrideClearance = ElkLayoutClearance.Current;
+        if (overrideClearance > 0d)
+        {
+            minClearance = overrideClearance;
+        }
 
         // Scan routed edges for horizontal segments with under-node or alongside violations.
         var gutterRequirements = new Dictionary<double, double>(); // gutterY → requiredShift