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.Mcp/NestSession.cs

@@ -8,6 +8,13 @@ namespace OpenNest.Mcp
         public List<Plate> Plates { get; } = new();
         public List<Drawing> Drawings { get; } = new();
 
+        /// <summary>
+        /// Session-default engine/strategy name used by nesting tools when a call does not pass
+        /// an explicit <c>engine</c> argument. Instance state only — the process-global legacy
+        /// registry is never read or written.
+        /// </summary>
+        public string DefaultEngineName { get; set; } = "Default";
+
         public Plate GetPlate(int index)
         {
             if (Nest != null && index < Nest.Plates.Count)