Complete ElkSharp document rendering cleanup and source decomposition

- Fix target-join (edge/4+edge/17): gateway face overflow redirect to left tip
- Fix under-node (edge/14,15,20): push-first corridor reroute instead of top corridor
- Fix boundary-slots (4->0): snap after gateway polish reordering
- Fix gateway corner diagonals (2->0): post-pipeline straightening pass
- Fix gateway interior adjacent: polygon-aware IsInsideNodeShapeInterior
- Fix gateway source face mismatch (2->0): per-edge redirect with lenient validation
- Fix gateway source scoring (5->0): per-edge scoring candidate application
- Fix edge-node crossing (1->0): push horizontal segment above blocking node
- Decompose 7 oversized files (~20K lines) into 55+ partials under 400 lines each
- Archive sprints 004 (document cleanup), 005 (decomposition), 007 (render speed)

All 44+ document-processing artifact assertions pass. Hybrid deterministic mode
documented as recommended path for LeftToRight layouts.

Tests verified: StraightExit 2/2, BoundarySlotOffenders 2/2, HybridDeterministicMode 3/3,
DocumentProcessingWorkflow artifact 1/1.

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

src/__Libraries/StellaOps.ElkSharp/ElkEdgeRouterIterative.WinnerRefinement.GatewayArtifacts.cs

@@ -93,6 +93,10 @@ internal static partial class ElkEdgeRouterIterative
             candidateEdges = ElkEdgePostProcessor.FinalizeGatewayBoundaryGeometry(candidateEdges, nodes, focusEdgeIds);
             candidateEdges = ElkEdgePostProcessor.NormalizeBoundaryAngles(candidateEdges, nodes);
             candidateEdges = ElkEdgePostProcessor.NormalizeSourceExitAngles(candidateEdges, nodes);
+            // Straighten any corner diagonals created by the face fix so they
+            // don't block promotion via the lexicographic IsBetterThan check
+            // (corners have higher priority than face mismatches).
+            candidateEdges = ElkEdgePostProcessor.StraightenGatewayCornerDiagonals(candidateEdges, nodes);
 
             if (!TryPromoteGatewayArtifactCandidate(current, candidateEdges, nodes, baselineArtifacts, out var promoted))
             {