fix(engine): make canonical-frame fills orientation-invariant

Part.Rotation is cumulative, so rebinding canonical parts with
CreateAtOrigin(original, p.Rotation) double-counted the drawing's own
rotation, and FromCanonical rotated each part about its Location, which
moved it off its slot and out of the work area. Add
CanonicalFrame.RebindToOriginal (rotation = part - original program
rotation, footprint aligned to the canonical part) and use it in the
three places that duplicated the old logic.

The MBR only fixes the frame modulo 90 degrees and nest results are not
90-degree symmetric (an L gave 56/43/42/42 parts by orientation).
CanonicalAngle.Compute now picks one of the four orientations from the
centroid offset; symmetric shapes keep the MBR orientation.

Fixes the three NestInvarianceTests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OpenNest.Engine/BestFit/BestFitResult.cs

@@ -70,19 +70,8 @@ namespace OpenNest.Engine.BestFit
         public List<Part> BuildSourceParts(Drawing drawing)
         {
             var parts = BuildCanonicalParts();
-            var sourceAngle = drawing?.Source?.Angle ?? 0.0;
 
-            for (var i = 0; i < parts.Count; i++)
-            {
-                var p = parts[i];
-                var rebound = Part.CreateAtOrigin(drawing, p.Rotation);
-                var delta = p.BoundingBox.Location - rebound.BoundingBox.Location;
-                rebound.Offset(delta);
-                rebound.UpdateBounds();
-                parts[i] = rebound;
-            }
-
-            return NormalizeToCutOrigin(CanonicalFrame.FromCanonical(parts, sourceAngle));
+            return NormalizeToCutOrigin(CanonicalFrame.RebindToOriginal(parts, drawing));
         }
 
         public Box GetCutBounds(List<Part> parts)