feat(engine): nest on outlined stock with the Irregular packer

NestPlateStock carries the outline; its usable material (less obstacles,
eroded by the largest edge spacing) becomes the FrontierPacker's
PackBoundary. The layout check and the runner's candidate validation
reject a part outside the outline or over an obstacle, area-based cost
and the area budget use the material area, salvage credits nothing, and
Rectangles, Circles, FlatBar and the fill strategies place nothing on it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

OpenNest.Engine/NestingEngines/Rectangles/RectanglesNestingEngine.cs

@@ -76,6 +76,8 @@ public sealed class RectanglesNestingEngine : INestingEngine
 
     private static bool FitsStock(NestPlateStock stock, BoxOrientation o)
     {
+        if (stock.Outline != null)
+            return false; // Boxes pack rectangles only.
         var work = stock.WorkArea;
         return o.Width <= work.Right - work.Left + SheetPacker.OverhangAllowance
             && o.Height <= work.Top - work.Bottom + SheetPacker.OverhangAllowance;