refactor(engine): route console and MCP nesting through named job engines

Console --engine now names a jobs engine for --autonest (solved once
through NestingEngineRegistry.Create and committed onto the plate) or a
built-in fill strategy for single-plate fill through the public
PlateFillService; unknown names exit with the valid choices instead of
consulting the process-global legacy registry. MCP nesting tools take an
explicit engine argument per call with the session default, never read
process-global active-engine state, and reject whole-job engine names on
single-plate fill tools. NestingEngineRegistry gains an explicit Create
(name) resolution; PlateFillService gains a public ResolveStrategy and a
plate-number Nest overload used by interactive callers.

OpenNest.Engine.Tests/PlateFillServiceTests.cs

@@ -102,6 +102,33 @@ public class PlateFillServiceTests
         Assert.All(parts, part => Assert.Same(drawing, part.BaseDrawing));
     }
 
+    [Fact]
+    public void Nest_RejectsUnknownStrategy_AndReportsPlateNumberInDetail()
+    {
+        var plate = new Plate(new Size(60, 80));
+        // Big enough (240 area x4 >= 10% of the 4800-area plate) to route through the fill
+        // pipeline, which is where progress detail is emitted.
+        var drawing = new Drawing("part", TestDrawingFactory.Rectangle(20, 12));
+        var items = new List<NestItem> { new() { Drawing = drawing, Quantity = 4 } };
+
+        Assert.Throws<NotSupportedException>(() =>
+            PlateFillService.Nest("StockLadder", plate, items, null, CancellationToken.None)
+        );
+        Assert.Throws<ArgumentNullException>(() =>
+            PlateFillService.Nest(null!, plate, items, null, CancellationToken.None)
+        );
+
+        // The plateNumber overload reports the caller's plate index like the legacy
+        // engine's PlateNumber did for interactive multi-plate loops.
+        var progress = new CapturingProgress();
+        var parts = PlateFillService.Nest("Default", plate, items, 3, progress, CancellationToken.None);
+
+        Assert.NotEmpty(parts);
+        Assert.NotEmpty(progress.Reports);
+        Assert.All(progress.Reports, report => Assert.Equal(3, report.PlateNumber));
+        Assert.Empty(plate.Parts); // proposed parts only; committing stays with the caller
+    }
+
     [Theory]
     [MemberData(nameof(StrategiesData))]
     public void FillItem_ReturnsNoParts_WhenTokenIsAlreadyCancelled(string strategy)