refactor: simplify MultiPlateNester by converting to instance class

- Convert static class to instance with private constructor; shared
  parameters (template, plateOptions, salvageRate, minRemnantSize,
  progress, token) become fields, eliminating parameter threading
  across all private methods (10→3 params on Nest entry point stays
  unchanged; private methods drop from 7-9 params to 1-2)
- Extract FillAndPlace helper consolidating the repeated
  clone→fill→add-to-plate→deduct-quantity pattern (was duplicated
  in 4 call sites)
- Merge FindScrapZones/FindViableRemnants (98% duplicate) into single
  FindRemnants(plate, minRemnantSize, scrapOnly) method
- Extract ScoreZone helper and collapse duplicate normal/rotated
  orientation checks into single conditional
- Extract CreateNewPlateResult helper for repeated PlateResult
  construction + PlateOption lookup pattern

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

OpenNest.Tests/Engine/MultiPlateNesterTests.cs

@@ -140,7 +140,7 @@ public class MultiPlateNesterTests
     }
 
     [Fact]
-    public void FindScrapZones_ReturnsOnlyScrapRemnants()
+    public void FindRemnants_ScrapOnly_ReturnsOnlyScrapRemnants()
     {
         // 96x48 plate with a 70x40 part placed at origin
         var plate = new Plate(96, 48) { PartSpacing = 0.25 };
@@ -148,7 +148,7 @@ public class MultiPlateNesterTests
         var part = new Part(drawing);
         plate.Parts.Add(part);
 
-        var scrap = MultiPlateNester.FindScrapZones(plate, 12.0);
+        var scrap = MultiPlateNester.FindRemnants(plate, 12.0, scrapOnly: true);
 
         // All returned zones should have both dims < 12
         foreach (var zone in scrap)