refactor(packing): move maximal-rectangles sheet into shared rectangle packing

MaxRectsSheet only tracks free rectangles and has no job dependencies, so it
moves from the Rectangles engine into OpenNest.Engine.RectanglePacking where
the interactive fill packer can use it. No behavior change.

OpenNest.Engine/NestingEngines/Rectangles/SheetPacker.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using OpenNest.Engine.Jobs;
+using OpenNest.Engine.RectanglePacking;
 using OpenNest.Geometry;
 
 namespace OpenNest.Engine.NestingEngines.Rectangles;