fix(cutting): keep clamp zones on cutting-plan previews

Plan Cutting previews were built without the plate's clamp zones, so
machine clamps disappeared from the canvas while inspecting a proposal.
Copy the plate's immutable zones onto the detached preview; it does not
follow the rail, so it never re-resolves.

Review finding 8, Project Memory 4e5f566d.

OpenNest.Engine/CuttingPlanning/CuttingPlanBatch.cs

@@ -336,6 +336,9 @@ public sealed class CuttingPlanProposal
             PartSpacing = source.PartSpacing,
             EdgeSpacing = source.EdgeSpacing,
         };
+        // Machine clamps stay visible while inspecting the proposal; zones are immutable.
+        foreach (var zone in source.ClampZones)
+            preview.ClampZones.Add(zone);
         foreach (var proposal in planned.Result.ProposedOrder)
         {
             var part = (Part)proposal.SourcePart.Clone();

OpenNest.Tests/CuttingPlanning/ClampZonePreviewTests.cs

@@ -0,0 +1,22 @@
+using OpenNest.Engine.CuttingPlanning;
+
+namespace OpenNest.Tests.CuttingPlanning;
+
+/// <summary>Machine clamps stay visible on a cutting-plan preview of a clamped plate.</summary>
+public class ClampZonePreviewTests
+{
+    [Fact]
+    public void PreviewCarriesThePlatesClampZones()
+    {
+        var plate = BestEffortCuttingPlanTests.Plate(BestEffortCuttingPlanTests.TouchingContours());
+        plate.SetClampRail(new ClampRail(1.5, 0.625, new[] { 6.0, 21 }));
+
+        var proposal = CuttingPlanBatch.Capture([plate], ExplicitContourTests.Parameters(), false).Plan();
+        var preview = proposal.BuildPreview(0);
+
+        Assert.NotNull(preview);
+        Assert.Equal(plate.ClampZones, preview.ClampZones); // same immutable zones, same order
+        Assert.Equal(2, preview.ClampZones.Count);
+        Assert.Null(preview.ClampRail); // a detached display copy never re-resolves
+    }
+}