feat(engine): add FixedStrategyNestingEngine adapting IPlateNester strategies to INestingEngine

Implements a sealed adapter class that forces a fixed IPlateNester strategy onto
any NestJob, overriding the job's own PlacementStrategy while preserving MaxPlates.
Delegates all multi-plate allocation and stock selection to NestJobRunner.

This allows single-plate nesting strategies to compete as full whole-job
INestingEngine solvers in benchmarks, enabling comparative performance testing
of placement algorithms across various job configurations.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

OpenNest.Engine/Jobs/FixedStrategyNestingEngine.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+
+namespace OpenNest;
+
+/// <summary>
+/// Adapts one fixed IPlateNester strategy to the whole-job INestingEngine contract, so it can compete
+/// as a full job solver alongside model-submitted engines. Delegates all multi-plate/size selection to
+/// NestJobRunner; only the placement strategy key is forced, overriding whatever the job itself declared.
+/// </summary>
+public sealed class FixedStrategyNestingEngine : INestingEngine
+{
+    private readonly string strategy;
+    private readonly NestJobRunner runner = new(PlateNesterFactory.Create);
+
+    public FixedStrategyNestingEngine(string strategy)
+    {
+        if (string.IsNullOrWhiteSpace(strategy))
+            throw new ArgumentException("Strategy cannot be null or whitespace.", nameof(strategy));
+        this.strategy = strategy;
+    }
+
+    public NestJobResult Solve(NestJob job, IProgress<NestJobProgress> progress = null, CancellationToken token = default)
+    {
+        ArgumentNullException.ThrowIfNull(job);
+        var forced = new NestJob(job.Parts, job.Plates, new NestJobOptions(strategy, job.Options.MaxPlates));
+        return runner.Solve(forced, progress, token);
+    }
+}