fix(ui): compute polylabel once from unrotated drawing

Compute the label point from the base drawing's unrotated program
and rotate the cached point instead of recomputing on each rotation.
Prevents label jitter caused by arc discretization differences.

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

OpenNest/LayoutPart.cs

@@ -46,16 +46,7 @@ namespace OpenNest
             Color = part.BaseDrawing.Color;
         }
 
-        private bool _isDirty;
-        internal bool IsDirty
-        {
-            get => _isDirty;
-            set
-            {
-                _isDirty = value;
-                if (value) _labelPoint = null;
-            }
-        }
+        internal bool IsDirty { get; set; }
 
         public bool IsSelected { get; set; }
         
@@ -119,14 +110,14 @@ namespace OpenNest
 
         private Vector ComputeLabelPoint()
         {
-            var entities = ConvertProgram.ToGeometry(BasePart.Program);
+            var entities = ConvertProgram.ToGeometry(BasePart.BaseDrawing.Program);
             var nonRapid = entities.Where(e => e.Layer != SpecialLayers.Rapid).ToList();
 
             var shapes = ShapeBuilder.GetShapes(nonRapid);
 
             if (shapes.Count == 0)
             {
-                var bbox = BasePart.Program.BoundingBox();
+                var bbox = BasePart.BaseDrawing.Program.BoundingBox();
                 return new Vector(bbox.Location.X + bbox.Width / 2, bbox.Location.Y + bbox.Length / 2);
             }
 
@@ -151,9 +142,10 @@ namespace OpenNest
             Path.Transform(plateView.Matrix);
 
             _labelPoint ??= ComputeLabelPoint();
+            var rotatedLabel = _labelPoint.Value.Rotate(BasePart.Rotation);
             var labelPt = new PointF(
-                (float)(_labelPoint.Value.X + BasePart.Location.X),
-                (float)(_labelPoint.Value.Y + BasePart.Location.Y));
+                (float)(rotatedLabel.X + BasePart.Location.X),
+                (float)(rotatedLabel.Y + BasePart.Location.Y));
             var pts = new[] { labelPt };
             plateView.Matrix.TransformPoints(pts);
             _labelScreenPoint = pts[0];