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/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}.")
+        };
+    }
+}