feat: implement PlateManager sentinel, reactive subscriptions, and batch ops (Tasks 3-5)

- EnsureSentinel() maintains exactly one trailing empty plate, suppressing navigation events during mutation
- Reactive tail subscriptions (PartAdded/PartRemoved on last two plates) call EnsureSentinel automatically; re-subscribed after each plate list change
- BeginBatch()/EndBatch() defers sentinel enforcement during bulk operations
- GetOrCreateEmpty() returns or creates an empty plate; RemoveCurrent() removes the current plate with index clamping; CanRemoveCurrent guards deletion
- 13 new tests (30 total PlateManager tests), all passing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Tests/PlateManagerTests.cs

@@ -205,4 +205,191 @@ public class PlateManagerTests
         nest.CreatePlate();
         Assert.False(eventFired);
     }
+
+    // Task 3: Sentinel plate invariant
+
+    [Fact]
+    public void EnsureSentinel_EmptyNest_CreatesOnePlate()
+    {
+        var nest = CreateNest();
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        Assert.Equal(1, nest.Plates.Count);
+        Assert.Equal(0, nest.Plates[0].Parts.Count);
+    }
+
+    [Fact]
+    public void EnsureSentinel_LastPlateHasParts_CreatesNewEmpty()
+    {
+        var nest = CreateNest();
+        var plate = nest.CreatePlate();
+        plate.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        Assert.Equal(2, nest.Plates.Count);
+        Assert.Equal(0, nest.Plates[^1].Parts.Count);
+    }
+
+    [Fact]
+    public void EnsureSentinel_TwoTrailingEmpties_TrimsToOne()
+    {
+        var nest = CreateNest();
+        nest.CreatePlate();
+        nest.CreatePlate();
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        Assert.Equal(1, nest.Plates.Count);
+    }
+
+    [Fact]
+    public void EnsureSentinel_PreservesCurrentIndex()
+    {
+        var nest = CreateNest();
+        var plate = nest.CreatePlate();
+        plate.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        Assert.Equal(0, mgr.CurrentIndex);
+        Assert.Same(plate, mgr.CurrentPlate);
+    }
+
+    // Task 4: Reactive tail-plate subscriptions
+
+    [Fact]
+    public void PartAddedToLastPlate_CreatesSentinel()
+    {
+        var nest = CreateNest();
+        nest.CreatePlate();
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        var initialCount = nest.Plates.Count;
+        nest.Plates[^1].Parts.Add(MakePart());
+        Assert.Equal(initialCount + 1, nest.Plates.Count);
+        Assert.Equal(0, nest.Plates[^1].Parts.Count);
+    }
+
+    [Fact]
+    public void PartRemovedFromSecondToLast_TrimsExtraEmpty()
+    {
+        var nest = CreateNest();
+        var plate = nest.CreatePlate();
+        plate.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        Assert.Equal(2, nest.Plates.Count);
+        nest.Plates[0].Parts.RemoveAt(0);
+        Assert.Equal(1, nest.Plates.Count);
+    }
+
+    [Fact]
+    public void ReactiveSubscription_ResubscribesAfterPlateListChange()
+    {
+        var nest = CreateNest();
+        var plate1 = nest.CreatePlate();
+        plate1.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+        nest.Plates[^1].Parts.Add(MakePart());
+        Assert.Equal(3, nest.Plates.Count);
+        nest.Plates[^1].Parts.Add(MakePart());
+        Assert.Equal(4, nest.Plates.Count);
+        Assert.Equal(0, nest.Plates[^1].Parts.Count);
+    }
+
+    // Task 5: Batch mode and plate operations
+
+    [Fact]
+    public void BeginBatch_DefersSentinelEnforcement()
+    {
+        var nest = CreateNest();
+        var plate = nest.CreatePlate();
+        plate.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+
+        mgr.BeginBatch();
+        nest.Plates[^1].Parts.Add(MakePart());
+        Assert.Equal(2, nest.Plates.Count);
+    }
+
+    [Fact]
+    public void EndBatch_EnforcesSentinelAndFiresEvents()
+    {
+        var nest = CreateNest();
+        var plate = nest.CreatePlate();
+        plate.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+
+        mgr.BeginBatch();
+        nest.Plates[^1].Parts.Add(MakePart());
+
+        var listChangedCount = 0;
+        mgr.PlateListChanged += (s, e) => listChangedCount++;
+
+        mgr.EndBatch();
+
+        Assert.Equal(3, nest.Plates.Count);
+        Assert.Equal(0, nest.Plates[^1].Parts.Count);
+        Assert.True(listChangedCount > 0);
+    }
+
+    [Fact]
+    public void GetOrCreateEmpty_ReturnsSentinelWhenEmpty()
+    {
+        var nest = CreateNest();
+        var plate1 = nest.CreatePlate();
+        plate1.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.EnsureSentinel();
+
+        var result = mgr.GetOrCreateEmpty();
+        Assert.Same(nest.Plates[^1], result);
+        Assert.Equal(0, result.Parts.Count);
+    }
+
+    [Fact]
+    public void GetOrCreateEmpty_NoEmptyPlate_CreatesNew()
+    {
+        var nest = CreateNest();
+        var plate1 = nest.CreatePlate();
+        plate1.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+
+        var result = mgr.GetOrCreateEmpty();
+        Assert.Equal(0, result.Parts.Count);
+    }
+
+    [Fact]
+    public void RemoveCurrent_RemovesPlateAndClampsIndex()
+    {
+        var nest = CreateNest();
+        var plate1 = nest.CreatePlate();
+        plate1.Parts.Add(MakePart());
+        var plate2 = nest.CreatePlate();
+        plate2.Parts.Add(MakePart());
+        using var mgr = new PlateManager(nest);
+        mgr.LoadLast();
+
+        mgr.RemoveCurrent();
+
+        Assert.Equal(1, nest.Plates.Count);
+        Assert.Equal(0, mgr.CurrentIndex);
+    }
+
+    [Fact]
+    public void CanRemoveCurrent_OnlyIfMultiplePlatesAndHasParts()
+    {
+        var nest = CreateNest();
+        var plate1 = nest.CreatePlate();
+        using var mgr = new PlateManager(nest);
+
+        Assert.False(mgr.CanRemoveCurrent);
+
+        plate1.Parts.Add(MakePart());
+        Assert.False(mgr.CanRemoveCurrent);
+
+        nest.CreatePlate();
+        Assert.True(mgr.CanRemoveCurrent);
+    }
 }