Fix entry-angle violations and add boundary-first routing infrastructure

The short-stub fallback in NormalizeExitPath fixes 2 entry-angle violations
(edge/7, edge/27) that persisted because the default long-stub normalization
created horizontal segments crossing nodes in occupied Y-bands. When the long
stub fails HasClearSourceExitSegment, the normalizer now tries a 24px short
stub that creates a perpendicular dog-leg exit avoiding the blocking node.

Also adds boundary-first routing infrastructure (not yet active in the main
path) including global boundary slot pre-computation, A* routing with
pre-assigned slots, coordinated cluster repair with net-total promotion
criterion, and gateway target approach overshoot clipping. The net-total
criterion (CountTotalHardViolations) is proven to reduce violations from
10 to 7 but requires expensive BuildFinalRestabilizedCandidate calls that
exceed the 15s speed budget.

Root cause analysis confirms the remaining 8 violations (3 gateway hooks,
1 target join, 1 shared lane, 3 under-node) are caused by Sugiyama node
placement creating routing corridors too narrow for clean edge routing.
The fix must happen upstream in node placement, not edge post-processing.

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

src/__Libraries/StellaOps.ElkSharp/ElkEdgeRouterIterative.BoundaryFirst.Verification.cs

@@ -0,0 +1,36 @@
+namespace StellaOps.ElkSharp;
+
+internal static partial class ElkEdgeRouterIterative
+{
+    private static ElkRoutedEdge[] ApplyBoundaryFirstVerification(
+        ElkRoutedEdge[] edges,
+        ElkRoutedEdge[] originalEdges,
+        ElkPositionedNode[] nodes,
+        ElkLayoutDirection direction,
+        double minLineClearance)
+    {
+        // Minimal structural safety only — the hybrid winner refinement
+        // handles remaining violations after boundary-first is promoted.
+        var result = ElkEdgePostProcessor.AvoidNodeCrossings(edges, nodes, direction);
+        result = ElkEdgePostProcessor.EliminateDiagonalSegments(result, nodes);
+        result = ElkEdgePostProcessorSimplify.SimplifyEdgePaths(result, nodes);
+        result = ElkEdgePostProcessorSimplify.TightenOuterCorridors(result, nodes);
+        if (HighwayProcessingEnabled)
+        {
+            result = ElkEdgeRouterHighway.BreakShortHighways(result, nodes);
+        }
+
+        // Normalize boundary geometry for the slot-pinned endpoints.
+        result = ElkEdgePostProcessor.NormalizeBoundaryAngles(result, nodes);
+        result = ElkEdgePostProcessor.NormalizeSourceExitAngles(result, nodes);
+
+        // Collector-specific structural repairs.
+        result = RestoreProtectedRepeatCollectorCorridors(result, originalEdges, nodes);
+
+        // Below-graph clamping and final crossing check.
+        result = ClampBelowGraphEdges(result, nodes);
+        result = ElkEdgePostProcessor.AvoidNodeCrossings(result, nodes, direction);
+
+        return result;
+    }
+}