fix(io): write an empty sheet whose rail differs only in removal time

The writer's sentinel rule compared zones only, so an empty sheet following
a rail that differs from the default only in clamp removal minutes was
dropped on save although ChangePlateDefaults treats it as configured stock.
The rule now also requires the sheet to follow the default rail itself.

PM 1469b4e0-dc59-416b-8118-560b726aa1e1

OpenNest.IO.Tests/ClampRailPersistenceTests.cs

@@ -470,6 +470,22 @@ public class ClampRailPersistenceTests
         Assert.Equal(edge, kept.EdgeSpacing.Left);
     }
 
+    // Delta review: a sheet whose rail differs from the defaults only in removal time is
+    // configured stock too (ChangePlateDefaults leaves it alone) and must be written.
+    [Fact]
+    public void EmptySheetOnARailWithOtherRemovalTimeIsKept()
+    {
+        var nest = new Nest("railed");
+        nest.PlateDefaults.Size = new Size(48, 96);
+        nest.SetClampRail(Machine());
+        var plate = nest.CreatePlate();
+        plate.SetClampRail(new ClampRail(1.5, 0.625, MachineCenters, removalMinutes: 7));
+
+        var kept = Assert.Single(RoundTrip(nest).Plates);
+
+        Assert.Equal(7, kept.ClampRail.RemovalMinutes);
+    }
+
     /// <summary>Removes a named property wherever it appears in the document, simulating a
     /// key an older writer never emitted. Rebuilds via <see cref="JsonNode"/> so the result
     /// is always valid JSON regardless of whether the property was first, last or only one

OpenNest.IO/NestWriter.cs

@@ -201,7 +201,8 @@ namespace OpenNest.IO
         /// <summary>
         /// True when a plate created from the saved defaults would be this sheet's clamp
         /// configuration: same size, quadrant, edge and part spacing, no last-resort
-        /// removal, and zones the saved rail recreates (C17: anything else is configured
+        /// removal, not following another rail (removal time included), and zones the
+        /// saved rail recreates (C17: anything else is configured
         /// stock and is written).
         /// </summary>
         private bool DefaultsRecreateClamps(Plate plate)
@@ -217,6 +218,7 @@ namespace OpenNest.IO
                 && plate.EdgeSpacing.Bottom == defaults.EdgeSpacing.Bottom
                 && plate.EdgeSpacing.Right == defaults.EdgeSpacing.Right
                 && plate.EdgeSpacing.Top == defaults.EdgeSpacing.Top
+                && (plate.ClampRail == null || Equals(plate.ClampRail, rail))
                 && rail.Matches(plate);
         }