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/Jobs/NestingEngineRegistryTests.cs

@@ -42,6 +42,27 @@ public class NestingEngineRegistryTests
         Assert.Equal(before, NestingEngineRegistry.AvailableEngines.Count);
     }
 
+    [Fact]
+    public void CreateResolvesKnownNamesCaseInsensitivelyAndRejectsUnknown()
+    {
+        Assert.NotNull(NestingEngineRegistry.Create("default"));
+        Assert.NotNull(NestingEngineRegistry.Create("Vertical Remnant"));
+        Assert.NotNull(NestingEngineRegistry.Create("stockladder"));
+
+        Assert.Throws<NotSupportedException>(() => NestingEngineRegistry.Create("Mystery Engine"));
+        Assert.Throws<ArgumentException>(() => NestingEngineRegistry.Create("  "));
+        Assert.Throws<ArgumentNullException>(() => NestingEngineRegistry.Create(null!));
+    }
+
+    [Fact]
+    public void CreateProducesAWorkingEngine()
+    {
+        var engine = NestingEngineRegistry.Create("Strip");
+        var result = engine.Solve(FiniteStockJobTests.Job(1));
+
+        Assert.Equal(NestJobStatus.Complete, result.Status);
+    }
+
     [Fact]
     public void LoadPluginsAgainstMissingDirectoryIsANoOp()
     {