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.IO/NestWriter.cs

@@ -41,6 +41,7 @@ namespace OpenNest.IO
 
             WriteNestJson(zipArchive);
             WritePrograms(zipArchive);
+            WriteEntities(zipArchive);
             WriteBestFits(zipArchive);
 
             return true;
@@ -312,6 +313,24 @@ namespace OpenNest.IO
             }
         }
 
+        private void WriteEntities(ZipArchive zipArchive)
+        {
+            foreach (var kvp in drawingDict.OrderBy(k => k.Key))
+            {
+                var drawing = kvp.Value;
+                if (drawing.SourceEntities == null || drawing.SourceEntities.Count == 0)
+                    continue;
+
+                var dto = EntitySerializer.ToDto(drawing.SourceEntities, drawing.SuppressedEntityIds);
+                var json = JsonSerializer.Serialize(dto, JsonOptions);
+
+                var entry = zipArchive.CreateEntry($"entities/entities-{kvp.Key}");
+                using var stream = entry.Open();
+                using var writer = new StreamWriter(stream, Encoding.UTF8);
+                writer.Write(json);
+            }
+        }
+
         private void WriteDrawing(Stream stream, Drawing drawing)
         {
             var program = drawing.Program;