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/Drawing.cs

@@ -2,6 +2,7 @@
 using OpenNest.CNC;
 using OpenNest.Converters;
 using OpenNest.Geometry;
+using System;
 using System.Collections.Generic;
 using System.Drawing;
 using System.Linq;
@@ -90,6 +91,18 @@ namespace OpenNest
 
         public List<Bend> Bends { get; set; } = new List<Bend>();
 
+        /// <summary>
+        /// Complete set of source entities with stable GUIDs.
+        /// Null when the drawing was created from G-code or an older nest file.
+        /// </summary>
+        public List<Entity> SourceEntities { get; set; }
+
+        /// <summary>
+        /// IDs of entities in <see cref="SourceEntities"/> that are suppressed (hidden).
+        /// Suppressed entities are excluded from the active Program but preserved for re-enabling.
+        /// </summary>
+        public HashSet<Guid> SuppressedEntityIds { get; set; } = new HashSet<Guid>();
+
         public double Area { get; protected set; }
 
         public void UpdateArea()