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/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);
             }