refactor(engines): move plugin engines into an Engines/ subfolder

Plugin engines (Opus55, Qwen, Terra) each add two projects at the repo
root, and more are coming; at a dozen they would outnumber the core
projects. They are also a different kind of thing: out-of-solution,
runtime-loaded plugins. Grouping them under Engines/ keeps the root
readable.

Engines/Directory.Build.props now holds the shared TFM, nullable and
implicit-usings settings and the OpenNest.Engine reference, so a new
engine's csproj is nearly empty. The tests/ compile exclusion lives in
Directory.Build.targets because a removal in .props runs before the SDK
adds its default Compile glob and has no effect.

Build-Engines.ps1 replaces the per-README manual build-and-copy steps
for deploying engines into the benchmark's runtime Engines/ folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Engines/OpenNest.Engine.Terra/TerraNestingEngine.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Threading;
+using OpenNest.Engine.Jobs;
+
+namespace OpenNest.Engine.Terra;
+
+/// <summary>
+/// TODO: name and describe the actual placement strategy here (e.g. "skyline packer with
+/// greedy shelf assignment", "NFP-based sliding placement with simulated-annealing order
+/// search", etc). This must be an independently designed algorithm — see README.md.
+/// </summary>
+public sealed class TerraNestingEngine : INestingEngine
+{
+    public NestJobResult Solve(
+        NestJob job,
+        IProgress<NestJobProgress>? progress = null,
+        CancellationToken token = default
+    )
+    {
+        ArgumentNullException.ThrowIfNull(job);
+
+        // TODO: implement independent placement logic here.
+        //
+        // Do NOT call NestingEngineRegistry.Create(...), PlateNesterFactory, or any
+        // FixedStrategyNestingEngine / StockLadderNestingEngine instance from inside this
+        // method. Decide placements yourself using OpenNest.Core / OpenNest.Geometry
+        // primitives (Polygon, NoFitPolygon, Collision, ConvexHull, RotatingCalipers, etc).
+        //
+        // job.Parts        -> requested parts (PartGeometrySnapshot geometry, quantity, priority, rotation policy)
+        // job.Plates        -> candidate stock sheets (size, spacing, quadrant, quantity)
+        // job.Options       -> job-wide options
+        //
+        // Return a NestJobResult built from NestJobPlateResult (one per used sheet, holding
+        // ordered NestJobPlacement values), PartFulfillment (requested vs placed per part id),
+        // and StockUsage (sheets used per stock id).
+
+        throw new NotImplementedException(
+            "Terra nesting engine placement logic not yet implemented."
+        );
+    }
+}