fix(core): make a plate's removed-clamp set immutable

Plate.RemovedClamps exposed a HashSet (and every untouched plate shared one
static empty HashSet), so a downcast could change one plate's removal state
or every plate's at once without re-resolving zones. It is now an
ImmutableHashSet.

PM 6ded8ac4-84fd-4e8b-9a3b-6f41e494542c item (e)

OpenNest.Core/Plate.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Linq;
 using OpenNest.Collections;
 using OpenNest.Geometry;
@@ -141,7 +142,7 @@ namespace OpenNest
         /// </summary>
         public ClampRail ClampRail { get; private set; }
 
-        private static readonly IReadOnlySet<string> NoRemovedClamps = new HashSet<string>();
+        private static readonly IReadOnlySet<string> NoRemovedClamps = ImmutableHashSet<string>.Empty;
 
         /// <summary>
         /// Rail clamp ids taken off this sheet as a last resort (D8) so a part that could not
@@ -183,7 +184,7 @@ namespace OpenNest
             ClampRail = rail;
             RemovedClamps = rail == null || removedClamps == null
                 ? NoRemovedClamps
-                : new HashSet<string>(removedClamps, StringComparer.Ordinal);
+                : removedClamps.ToImmutableHashSet(StringComparer.Ordinal);
             ReplaceClampZones(ResolveRailZones());
         }
 

OpenNest.Tests/ClampRailTests.cs

@@ -382,4 +382,17 @@ public class ClampRailTests
         Assert.Equal(ClampRail.DefaultRemovalMinutes, plate.ClampRemovalMinutes);
         Assert.Contains("clamp-8", plate.RemovedClamps); // still listed (C13)
     }
+
+    // PM 6ded8ac4 (e): the removed set is read-only, even through a downcast, and an
+    // untouched plate does not share a mutable empty set with every other plate.
+    [Fact]
+    public void RemovedClampsCannotBeMutatedThroughTheCollection()
+    {
+        var removed = Sheet(48, 120);
+        removed.SetClampRail(Machine(), new[] { "clamp-3" });
+        var untouched = Sheet(48, 120);
+
+        Assert.True(((ICollection<string>)removed.RemovedClamps).IsReadOnly);
+        Assert.True(((ICollection<string>)untouched.RemovedClamps).IsReadOnly);
+    }
 }