refactor(engine): select desktop nesting through app-scoped jobs engine

Replace the MainForm/AutoNestForm engine combo bindings and every
desktop NestEngineRegistry call: selection now lives in app-scoped
EngineSelection addressing jobs engines in NestingEngineRegistry, the
combo lists the four built-in strategies (StockLadder stays out per the
frozen UI decision), and Engines/ plug-ins load through
NestingEngineRegistry.LoadPlugins. Whole-job fill routes through the
public PlateFillService with the selected strategy and plate number;
interactive group fill (PlateView) and area fill (ActionFillArea) go
through PlateFillService with identical accept/cancel preview behavior.
Multi-plate and size-search orchestrators receive the explicit strategy.
No desktop code reads or writes process-global engine state.

OpenNest/Actions/ActionFillArea.cs

@@ -6,6 +6,8 @@ using System.Threading.Tasks;
 using System.Windows.Forms;
 using OpenNest.Controls;
 using OpenNest.Engine;
+using OpenNest.Engine.Jobs.Placement;
+using OpenNest.Forms;
 
 namespace OpenNest.Actions
 {
@@ -46,13 +48,15 @@ namespace OpenNest.Actions
 
         private async void FillArea()
         {
+            var strategy = EngineSelection.FillStrategy;
             if (progress != null && cts != null)
             {
                 try
                 {
-                    var engine = NestEngineRegistry.Create(plateView.Plate);
                     var parts = await Task.Run(() =>
-                        engine.Fill(
+                        PlateFillService.FillItem(
+                            strategy,
+                            plateView.Plate,
                             new NestItem { Drawing = drawing },
                             SelectedArea,
                             progress,
@@ -69,8 +73,15 @@ namespace OpenNest.Actions
             }
             else
             {
-                var engine = NestEngineRegistry.Create(plateView.Plate);
+                var parts = PlateFillService.FillItem(
-                engine.Fill(new NestItem { Drawing = drawing }, SelectedArea);
+                    strategy,
+                    plateView.Plate,
+                    new NestItem { Drawing = drawing },
+                    SelectedArea,
+                    null,
+                    CancellationToken.None
+                );
+                plateView.Plate.Parts.AddRange(parts);
                 plateView.Invalidate();
             }
 

OpenNest/Controls/PlateView.cs

@@ -16,6 +16,7 @@ using OpenNest.Geometry;
 using OpenNest.Math;
 using Timer = System.Timers.Timer;
 using OpenNest.Engine;
+using OpenNest.Engine.Jobs.Placement;
 
 namespace OpenNest.Controls
 {
@@ -602,11 +603,18 @@ namespace OpenNest.Controls
 
             try
             {
-                var engine = NestEngineRegistry.Create(Plate);
+                var strategy = EngineSelection.FillStrategy;
                 var spacing = Plate.PartSpacing;
                 var parts = await Task.Run(() =>
                 {
-                    var result = engine.Fill(groupParts, workArea, progress, cts.Token);
+                    var result = PlateFillService.FillGroup(
+                        strategy,
+                        Plate,
+                        groupParts,
+                        workArea,
+                        progress,
+                        cts.Token
+                    );
                     Compactor.Settle(result, workArea, spacing);
                     return result;
                 });

OpenNest/Forms/AutoNestForm.cs

@@ -87,9 +87,9 @@ namespace OpenNest.Forms
 
         private void LoadEngines()
         {
-            foreach (var engine in NestEngineRegistry.AvailableEngines)
+            foreach (var name in EngineSelection.UiEngineNames)
-                engineComboBox.Items.Add(engine.Name);
+                engineComboBox.Items.Add(name);
-            engineComboBox.SelectedItem = NestEngineRegistry.ActiveEngineName;
+            engineComboBox.SelectedItem = EngineSelection.EngineName;
         }
 
         private void SetupPartsGrid()

OpenNest/Forms/EngineSelection.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using OpenNest.Engine.Jobs;
+using OpenNest.Engine.Jobs.Placement;
+
+namespace OpenNest.Forms
+{
+    /// <summary>
+    /// App-scoped nesting-engine selection — the desktop replacement for the process-global
+    /// NestEngineRegistry.ActiveEngineName. The selected name addresses a whole-job
+    /// INestingEngine resolved through NestingEngineRegistry at call time; single-plate
+    /// interactive fill uses FillStrategy, which maps a built-in engine to its placement
+    /// strategy and falls back to Default for jobs-only engines (StockLadder, plug-ins).
+    /// </summary>
+    public static class EngineSelection
+    {
+        public const string DefaultEngineName = "Default";
+
+        /// <summary>Registered jobs engine deliberately kept out of the desktop combo.</summary>
+        public const string HiddenEngineName = "StockLadder";
+
+        private static string engineName = DefaultEngineName;
+
+        public static string EngineName
+        {
+            get { return engineName; }
+            set
+            {
+                engineName = string.IsNullOrWhiteSpace(value)
+                    ? DefaultEngineName
+                    : value.Trim();
+            }
+        }
+
+        /// <summary>Desktop combo contents: registered jobs engines minus StockLadder.</summary>
+        public static IEnumerable<string> UiEngineNames =>
+            NestingEngineRegistry.AvailableEngines
+                .Where(e => !e.Name.Equals(HiddenEngineName, StringComparison.OrdinalIgnoreCase))
+                .Select(e => e.Name)
+                .ToList();
+
+        /// <summary>Placement strategy for single-plate fill: the selection when it names a
+        /// built-in strategy, otherwise Default.</summary>
+        public static string FillStrategy => IsFillStrategy(engineName) ? engineName : DefaultEngineName;
+
+        public static bool IsFillStrategy(string name) =>
+            !string.IsNullOrWhiteSpace(name)
+            && PlateFillService.BuiltInStrategies.Any(
+                s => s.Equals(name.Trim(), StringComparison.OrdinalIgnoreCase)
+            );
+    }
+}

OpenNest/Forms/MainForm.cs

@@ -17,6 +17,9 @@ using OpenNest.Gpu;
 using OpenNest.IO;
 using OpenNest.Properties;
 using OpenNest.Engine;
+using OpenNest.Engine.Jobs;
+using OpenNest.Engine.Jobs.Adapters;
+using OpenNest.Engine.Jobs.Placement;
 
 namespace OpenNest.Forms
 {
@@ -68,16 +71,19 @@ namespace OpenNest.Forms
             //if (GpuEvaluatorFactory.GpuAvailable)
             //    BestFitCache.CreateSlideComputer = () => GpuEvaluatorFactory.CreateSlideComputer();
 
+            // Jobs-side plug-in discovery: INestingEngine implementations are registered per
+            // assembly/type with the same per-DLL isolation as before. Binary plug-ins derived
+            // from the legacy NestEngineBase no longer load here after the Phase-4 removal.
             var enginesDir = Path.Combine(Application.StartupPath, "Engines");
-            NestEngineRegistry.LoadPlugins(enginesDir);
+            NestingEngineRegistry.LoadPlugins(enginesDir);
 
             OptionsForm.ApplyDisabledStrategies();
             ColorSchemeRegistry.ApplyActiveFromSettings();
 
-            foreach (var engine in NestEngineRegistry.AvailableEngines)
+            foreach (var name in EngineSelection.UiEngineNames)
-                engineComboBox.Items.Add(engine.Name);
+                engineComboBox.Items.Add(name);
 
-            engineComboBox.SelectedItem = NestEngineRegistry.ActiveEngineName;
+            engineComboBox.SelectedItem = EngineSelection.EngineName;
             engineComboBox.SelectedIndexChanged += EngineComboBox_SelectedIndexChanged;
         }
 
@@ -334,7 +340,7 @@ namespace OpenNest.Forms
         private void EngineComboBox_SelectedIndexChanged(object sender, EventArgs e)
         {
             if (engineComboBox.SelectedItem is string name)
-                NestEngineRegistry.ActiveEngineName = name;
+                EngineSelection.EngineName = name;
         }
 
         private void UpdateLocationMode()
@@ -1009,7 +1015,7 @@ namespace OpenNest.Forms
 
             if (form.EngineName != null)
             {
-                NestEngineRegistry.ActiveEngineName = form.EngineName;
+                EngineSelection.EngineName = form.EngineName;
                 engineComboBox.SelectedItem = form.EngineName;
             }
 
@@ -1118,6 +1124,7 @@ namespace OpenNest.Forms
                     SortOrder = sortOrder,
                     MinRemnantSize = minRemnantSize,
                     AllowPlateCreation = allowPlateCreation,
+                    Strategy = EngineSelection.FillStrategy,
                 };
 
                 var result = await Task.Run(() =>
@@ -1199,7 +1206,8 @@ namespace OpenNest.Forms
                         salvageRate,
                         plate,
                         progress,
-                        token
+                        token,
+                        EngineSelection.FillStrategy
                     )
                 );
 
@@ -1223,10 +1231,19 @@ namespace OpenNest.Forms
             }
             else
             {
-                var engine = NestEngineRegistry.Create(plate);
+                // Same preview flow as before: fill the current plate's remaining demand with the
-                engine.PlateNumber = plateIndex;
+                // selected strategy, then commit the returned parts. App-scoped selection — the
-
+                // process-global engine registry is never consulted.
-                nestParts = await Task.Run(() => engine.Nest(items, progress, token));
+                nestParts = await Task.Run(() =>
+                    PlateFillService.Nest(
+                        EngineSelection.FillStrategy,
+                        plate,
+                        items,
+                        plateIndex,
+                        progress,
+                        token
+                    )
+                );
             }
 
             activeForm.PlateView.ClearPreviewParts();