test(expand): bound pinned relaxation passes

OpenNest.Engine.Tests/Fill/ExpanderTests.cs

@@ -623,6 +623,34 @@ public class ExpanderTests
         Assert.Equal(0, wall.Location.X);
     }
 
+    [Fact]
+    public void Separate_PinnedBetweenFixedParts_StopsAfterOneNoProgressPass()
+    {
+        var plate = MakePlate(20, 20, edge: 0);
+        var pinned = AddSquare(plate, 8, 8);
+        AddSquare(plate, 3.8, 8);
+        AddSquare(plate, 12.2, 8);
+        AddSquare(plate, 8, 3.8);
+        AddSquare(plate, 8, 12.2);
+        var passes = 0;
+        var initial = pinned.Location;
+
+        var (converged, poses, violations) = Expander.SeparateObserved(
+            new List<Part> { pinned }, plate, spacing: 1,
+            checkpoint =>
+            {
+                if (checkpoint == Expander.Checkpoint.AfterPairPass)
+                    passes++;
+            }, maxIterations: 10000
+        );
+
+        Assert.False(converged);
+        Assert.NotEmpty(violations);
+        Assert.Equal(1, passes); // Not a MaxIterations spin or wall-clock assertion.
+        Assert.Equal(initial, poses[0]);
+        Assert.Equal(initial, pinned.Location);
+    }
+
     [Fact]
     public void Expand_WallsAndUnselectedPairs_StayExactlyAtClearance()
     {

OpenNest.Engine/Fill/Expander.cs

@@ -77,6 +77,7 @@ namespace OpenNest.Engine.Fill
         {
             AfterScratchMove,
             AfterSuccessfulProbe,
+            AfterPairPass,
             BeforeApply,
         }
 
@@ -307,6 +308,15 @@ namespace OpenNest.Engine.Fill
             return Separate(context, positions, spacing, maxIterations, token, null);
         }
 
+        internal static (bool Converged, List<Vector> Positions, List<Violation> Violations) SeparateObserved(
+            List<Part> selected, Plate plate, double spacing,
+            Action<Checkpoint> observer, int maxIterations = 100,
+            CancellationToken token = default)
+        {
+            var context = SeparationContext.Prepare(selected, plate, token);
+            return Separate(context, context.Positions(), spacing, maxIterations, token, observer);
+        }
+
         private static void Apply(List<Part> selected, List<Vector> from, List<Vector> to)
         {
             // Only movers occupy [0, selected.Count); walls never move.
@@ -435,6 +445,9 @@ namespace OpenNest.Engine.Fill
                     // the next pass. The final sweep remeasures all pairs.
                 }
 
+                observer?.Invoke(Checkpoint.AfterPairPass);
+                if (token.IsCancellationRequested)
+                    return (false, start, violations);
                 if (moved < epsMove)
                     break;
             }