fix: add overlap detection safety net for pair tiling

Shape.OffsetOutward produces inward offsets for certain rotated polygons,
causing geometry-aware copy distances to be too small and placing
overlapping parts. Root cause is in the offset winding direction
detection — this commit adds safety nets while that is investigated.

- FillLinear.FillGrid: detect bbox overlaps after geometry-aware tiling,
  fall back to bbox-based spacing when overlaps found
- FillExtents.RepeatColumns: detect overlaps after Compactor computes
  copy distance, fall back to columnWidth + spacing
- PairFiller/StripeFiller remnant fills: use FillLinear directly instead
  of spawning full engine pipeline (avoids strategies with the bug)
- Add PairOverlapDiagnosticTests reproducing the issue
- MCP config: use shadow-copy wrapper for dev hot-reload

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

OpenNest.Engine/Fill/PairFiller.cs

@@ -321,9 +321,19 @@ namespace OpenNest.Engine.Fill
                 return cachedResult;
             }
 
-            var remnantEngine = NestEngineRegistry.Create(plate);
-            var item = new NestItem { Drawing = drawing };
-            var parts = remnantEngine.Fill(item, remnantBox, null, token);
+            var filler = new FillLinear(remnantBox, partSpacing);
+            List<Part> parts = null;
+
+            foreach (var angle in new[] { 0.0, Angle.HalfPI })
+            {
+                token.ThrowIfCancellationRequested();
+                var result = FillHelpers.FillWithDirectionPreference(
+                    dir => filler.Fill(drawing, angle, dir),
+                    null, comparer, remnantBox);
+
+                if (result != null && result.Count > (parts?.Count ?? 0))
+                    parts = result;
+            }
 
             Debug.WriteLine($"[PairFiller] Remnant: {parts?.Count ?? 0} parts in " +
                 $"{remnantBox.Width:F2}x{remnantBox.Length:F2}");