refactor: CutOff uses Dictionary<Part, Entity> instead of index-based list

Replace CutOff.BuildPerimeterCache (List<Shape>) with Plate.BuildPerimeterCache
(Dictionary<Part, Entity>) throughout. Consolidate two Regenerate overloads into
a single method with optional cache parameter. Fix Shape intersection bug where
non-intersecting entities added spurious Vector.Zero points to results.

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

OpenNest.Core/Geometry/Intersect.cs

@@ -249,9 +249,8 @@ namespace OpenNest.Geometry
 
             foreach (var geo in shape.Entities)
             {
-                List<Vector> pts3;
-                geo.Intersects(line, out pts3);
-                pts.AddRange(pts3);
+                if (geo.Intersects(line, out var pts3))
+                    pts.AddRange(pts3);
             }
 
             return pts.Count > 0;