feat: serialize plate optimizer settings in nest files

Add PlateOptions and SalvageRate properties to the Nest class and
round-trip them through NestWriter/NestReader via a new PlateOptionDto.

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

OpenNest.Core/Nest.cs

@@ -1,6 +1,7 @@
 using OpenNest.Collections;
 using OpenNest.Geometry;
 using System;
+using System.Collections.Generic;
 
 namespace OpenNest
 {
@@ -51,6 +52,10 @@ namespace OpenNest
 
         public PlateSettings PlateDefaults { get; set; }
 
+        public List<PlateOption> PlateOptions { get; set; } = new();
+
+        public double SalvageRate { get; set; } = 0.5;
+
         public Plate CreatePlate()
         {
             var plate = PlateDefaults.CreateNew();