fix: normalize best-fit pairs to landscape and fix viewer size swap

Normalize pair bounding box to landscape (width >= height) in
PairEvaluator for consistent display and filtering. Fix
BestFitViewerForm where BoundingWidth/BoundingHeight were passed
in the wrong order to the plate Size constructor.

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

OpenNest/Forms/BestFitViewerForm.cs

@@ -291,8 +291,8 @@ namespace OpenNest.Forms
             cell.PartColor = partColor;
             cell.Dock = DockStyle.Fill;
             cell.Plate.Size = new Geometry.Size(
-                result.BoundingHeight,
-                result.BoundingWidth);
+                result.BoundingWidth,
+                result.BoundingHeight);
 
             var parts = result.BuildParts(drawing);