feat: add _Template and New-Engine.ps1 for starting new engines

Each engine-building run started from a hand-copied scaffold (Qwen's),
which carried that engine's name and stale notes. _Template holds the
generic scaffold with a __NAME__ placeholder, and New-Engine.ps1 stamps
out a named copy. The template's starter tests go through the
benchmark's NestValidator, so a model gets a real pass/fail target
instead of a plumbing-only check; they were verified to pass against
Opus55.

Directory.Build.props now also detects when it sits in an Engines/
folder inside an OpenNest checkout, so an engine stamped there with
-IncludeBuildFiles builds without passing OpenNestRoot.

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

_Template/__NAME__NestingEngine.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Threading;
+using OpenNest.Engine.Jobs;
+
+namespace OpenNest.Engine.__NAME__;
+
+/// <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 __NAME__NestingEngine : 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, PlateFillService,
+        // or any built-in INestingEngine, and do not run several and keep the best. The
+        // Fill/ and pattern components (FillLinear, PairFiller, PatternTiler, Compactor, ...)
+        // and OpenNest.Core geometry ARE fair game as tools; the decisions are yours.
+        //
+        // job.Parts    -> requested parts (PartGeometrySnapshot geometry, quantity, priority, rotation policy)
+        // job.Plates   -> candidate stock sheets (size, spacing, edge 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("__NAME__ nesting engine placement logic not yet implemented.");
+    }
+}