feat(engine): scaffold independent nesting engine plugins for Sonnet5, Terra, Qwen

Three standalone INestingEngine plugin projects (Solve() unimplemented,
throws NotImplementedException), each outside OpenNest.sln and discovered
at runtime via NestingEngineRegistry.LoadPlugins, same pattern as
OpenNest.Engine.Aurora. Each README spells out that Solve() must be an
independently designed placement algorithm and must never call/wrap/select
over the built-in engines (StockLadderNestingEngine, FixedStrategyNestingEngine,
PlateNesterFactory, NestingEngineRegistry).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OpenNest.Engine.Qwen/tests/OpenNest.Engine.Qwen.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
+    <PackageReference Include="xunit" Version="2.5.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+  </ItemGroup>
+  <ItemGroup>
+    <Using Include="Xunit" />
+    <ProjectReference Include="../OpenNest.Engine.Qwen.csproj" />
+    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
+  </ItemGroup>
+</Project>

OpenNest.Engine.Qwen/tests/QwenNestingEngineTests.cs

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+using System;
+using System.Collections.Generic;
+using OpenNest.Engine.Jobs;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Qwen.Tests;
+
+public class QwenNestingEngineTests
+{
+    [Fact]
+    public void SolveReturnsAResultForASingleSimplePart()
+    {
+        // TODO: replace with a real fixture once Solve() is implemented — this only
+        // proves the plumbing (project reference, constructor, interface) is wired up.
+        var engine = new QwenNestingEngine();
+
+        Assert.NotNull(engine);
+        Assert.IsAssignableFrom<INestingEngine>(engine);
+    }
+}