feat: add entity-based suppression with stable GUIDs

Entities now have a Guid Id for stable identity across edit sessions.
Drawing stores the full source entity set (SourceEntities) and a set of
suppressed entity IDs (SuppressedEntityIds), replacing the previous
SuppressedProgram approach. Unchecked entities in the converter are
suppressed rather than permanently removed, so they can be re-enabled
when editing drawings again.

Entities are serialized as JSON in the nest file (entities/entities-N)
alongside the existing G-code programs. Backward compatible with older
nest files that lack entity data.

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

OpenNest.Core/Geometry/Entity.cs

@@ -1,4 +1,5 @@
 using OpenNest.Math;
+using System;
 using System.Collections.Generic;
 using System.Drawing;
 
@@ -10,10 +11,16 @@ namespace OpenNest.Geometry
 
         protected Entity()
         {
+            Id = Guid.NewGuid();
             Layer = OpenNest.Geometry.Layer.Default;
             boundingBox = new Box();
         }
 
+        /// <summary>
+        /// Unique identifier for this entity, stable across edit sessions.
+        /// </summary>
+        public Guid Id { get; set; }
+
         /// <summary>
         /// Entity color (resolved from DXF ByLayer/ByBlock to actual color).
         /// </summary>