fix(engine): isolate job identity and engine selection

Replace name-based quantity deduction with reference-based drawing
identity in the engine paths the whole-job runner reaches
(NestEngineBase fill/pack, StripNestEngine deduction, RemnantFiller
ledger, IterativeShrinkFiller leftovers). Add instance-scoped
PlateNesterFactory that resolves built-in strategies without touching
the global NestEngineRegistry. Add identity and engine-selection tests.

44 net8.0 tests pass in Debug and Release; no new warnings.

OpenNest.Engine/Fill/IterativeShrinkFiller.cs

@@ -1,6 +1,7 @@
 using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -130,17 +131,13 @@ namespace OpenNest.Engine.Fill
 
             var placed = filler.FillItems(workItems, shrinkWrapper, token);
 
-            // Build leftovers: compare placed count to original quantities.
-            // RemnantFiller.FillItems does NOT mutate NestItem.Quantity.
+            // Build leftovers: compare placed count to original quantities by drawing
+            // reference. RemnantFiller.FillItems does NOT mutate NestItem.Quantity.
             var leftovers = new List<NestItem>();
             foreach (var item in items)
             {
-                var placedCount = 0;
-                foreach (var p in placed)
-                {
-                    if (p.BaseDrawing.Name == item.Drawing.Name)
-                        placedCount++;
-                }
+                var placedCount = placed.Count(p =>
+                    ReferenceEquals(p.BaseDrawing, item.Drawing));
 
                 if (item.Quantity <= 0)
                     continue; // unlimited items are always "satisfied" — no leftover

OpenNest.Engine/Fill/RemnantFiller.cs

@@ -58,20 +58,20 @@ namespace OpenNest.Engine.Fill
             return allParts;
         }
 
-        private static Dictionary<string, int> BuildLocalQuantities(List<NestItem> items)
+        private static Dictionary<Drawing, int> BuildLocalQuantities(List<NestItem> items)
         {
-            var localQty = new Dictionary<string, int>(items.Count);
+            var localQty = new Dictionary<Drawing, int>(items.Count, ReferenceEqualityComparer.Instance);
             foreach (var item in items)
-                localQty[item.Drawing.Name] = item.Quantity;
+                localQty[item.Drawing] = item.Quantity;
             return localQty;
         }
 
-        private static double FindMinItemDimension(List<NestItem> items, Dictionary<string, int> localQty)
+        private static double FindMinItemDimension(List<NestItem> items, Dictionary<Drawing, int> localQty)
         {
             var minDim = double.MaxValue;
             foreach (var item in items)
             {
-                if (localQty[item.Drawing.Name] <= 0)
+                if (localQty[item.Drawing] <= 0)
                     continue;
                 var bb = item.Drawing.Program.BoundingBox();
                 var dim = System.Math.Min(bb.Width, bb.Length);
@@ -84,7 +84,7 @@ namespace OpenNest.Engine.Fill
         private bool TryFillOneItem(
             List<NestItem> items,
             List<Box> freeBoxes,
-            Dictionary<string, int> localQty,
+            Dictionary<Drawing, int> localQty,
             Func<NestItem, Box, List<Part>> fillFunc,
             List<Part> allParts,
             CancellationToken token)
@@ -94,7 +94,7 @@ namespace OpenNest.Engine.Fill
                 if (token.IsCancellationRequested)
                     return false;
 
-                var qty = localQty[item.Drawing.Name];
+                var qty = localQty[item.Drawing];
                 if (qty <= 0)
                     continue;
 
@@ -110,7 +110,7 @@ namespace OpenNest.Engine.Fill
                     RemoveTopmostPart(placed);
 
                 allParts.AddRange(placed);
-                localQty[item.Drawing.Name] = System.Math.Max(0, qty - placed.Count);
+                localQty[item.Drawing] = System.Math.Max(0, qty - placed.Count);
 
                 // Add the envelope of all placed parts as a single obstacle
                 // rather than individual bounding boxes, preventing the

OpenNest.Engine/Jobs/PlateNesterFactory.cs

@@ -0,0 +1,23 @@
+using System;
+namespace OpenNest;
+
+/// <summary>
+/// Instance-scoped strategy resolution for the whole-job runner. The built-in strategies map
+/// to private engine factories; the process-global NestEngineRegistry (including plugin
+/// registrations and ActiveEngineName) is neither read nor modified. Unknown keys reject.
+/// </summary>
+public static class PlateNesterFactory
+{
+    public static IPlateNester Create(string strategy)
+    {
+        ArgumentNullException.ThrowIfNull(strategy);
+        return strategy switch
+        {
+            "Default" => new LegacyPlateNesterAdapter(plate => new DefaultNestEngine(plate)),
+            "Strip" => new LegacyPlateNesterAdapter(plate => new StripNestEngine(plate)),
+            "Vertical Remnant" => new LegacyPlateNesterAdapter(plate => new VerticalRemnantEngine(plate)),
+            "Horizontal Remnant" => new LegacyPlateNesterAdapter(plate => new HorizontalRemnantEngine(plate)),
+            _ => throw new NotSupportedException($"Unknown placement strategy: {strategy}.")
+        };
+    }
+}

OpenNest.Engine/NestEngineBase.cs

@@ -113,11 +113,11 @@ namespace OpenNest
                 {
                     allParts.AddRange(fillParts);
 
-                    // Deduct placed quantities
+                    // Deduct placed quantities by drawing reference, not name.
                     foreach (var item in fillItems)
                     {
                         var placed = fillParts.Count(p =>
-                            p.BaseDrawing.Name == item.Drawing.Name);
+                            ReferenceEquals(p.BaseDrawing, item.Drawing));
                         item.Quantity = System.Math.Max(0, item.Quantity - placed);
                     }
 
@@ -147,10 +147,11 @@ namespace OpenNest
                 {
                     allParts.AddRange(packParts);
 
+                    // Deduct placed quantities by drawing reference, not name.
                     foreach (var item in regularPackItems)
                     {
                         var placed = packParts.Count(p =>
-                            p.BaseDrawing.Name == item.Drawing.Name);
+                            ReferenceEquals(p.BaseDrawing, item.Drawing));
                         item.Quantity = System.Math.Max(0, item.Quantity - placed);
                     }
                 }

OpenNest.Engine/StripNestEngine.cs

@@ -128,13 +128,14 @@ namespace OpenNest
                 }
             }
 
-            // Deduct placed quantities from original items.
+            // Deduct placed quantities from original items by drawing reference.
             foreach (var item in items)
             {
                 if (item.Quantity <= 0)
                     continue;
 
-                var placed = allParts.Count(p => p.BaseDrawing.Name == item.Drawing.Name);
+                var placed = allParts.Count(p =>
+                    ReferenceEquals(p.BaseDrawing, item.Drawing));
                 item.Quantity = System.Math.Max(0, item.Quantity - placed);
             }