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>

CLAUDE.md

@@ -47,6 +47,7 @@ Nesting algorithms use the jobs-only API. `INestingEngine.Solve(NestJob)` return
 - **Placement boundary (`Jobs/Placement/`, `Jobs/Adapters/`)**: `DefaultPlateNester`, `StripPlateNester`, and `RemnantPlateNester` are built-ins with run-scoped private geometry. `PlateFillService` is the public single-plate proposal service for interactive fill/group/pack flows; it returns parts without mutating caller-owned plates. Job-path identity is reference-based rather than drawing name; `PlateOptimizer` retains name-based helpers and remains outside the runner path.
 - **Filler pipeline (`Jobs/Placement/Fillers/`)**: internal `DefaultPlateFiller`, `StripPlateFiller`, and policy-backed `RemnantPlateFiller` implement the standard single-plate geometry pipeline. `Default` runs the Linear, Pairs, RectBestFit, and Extents phases; remnant variants preserve their distinct comparer, direction, trim-axis, and angle-ordering policies.
 - **Engine registration**: `NestingEngineRegistry` holds whole-job `INestingEngine` implementations including the four fixed strategies and `StockLadder`. It loads plug-ins that implement `INestingEngine` and have a public parameterless constructor. Plug-ins for the removed single-plate inheritance API are not binary compatible.
+- **In-repo plugin engines (`Engines/`)**: each lives in `Engines/OpenNest.Engine.<Name>/` with an optional `tests/` subproject, outside `OpenNest.sln`. `Engines/Directory.Build.props` supplies the TFM, nullable/implicit usings, and the `OpenNest.Engine` reference, so an engine csproj only adds what is unique to it; `Engines/Directory.Build.targets` excludes `tests/**` from the engine compile (it must be a `.targets` file to run after the SDK's default Compile glob). `./Engines/Build-Engines.ps1 [-Engines Name1,Name2]` builds the benchmark and engines and deploys the DLLs into `OpenNest.Benchmark/bin/<Config>/net8.0/Engines/`. Add new engines here, not at the repo root.
 - **IFillComparer**: Interface enabling filler-specific scoring. `DefaultFillComparer` (count-then-density), `VerticalRemnantComparer` (minimize X-extent), and `HorizontalRemnantComparer` (minimize Y-extent) are grouped into `FillPolicy` on `FillContext`.
 - **Fill/** (`namespace OpenNest.Engine.Fill`): Fill algorithms — `FillLinear` (grid-based), `FillExtents` (extents-based pair tiling), `PairFiller` (interlocking pairs), `ShrinkFiller`, `RemnantFiller`/`RemnantFinder`, `Compactor` (post-fill gravity compaction), `FillScore` (lexicographic comparison: count > utilization > compactness), `Pattern`/`PatternTiler`, `PartBoundary`, `RotationAnalysis`, `AngleCandidateBuilder`, `BestCombination`, `AccumulatingProgress`.
 - **Strategies/** (`namespace OpenNest.Engine.Strategies`): Pluggable fill strategy layer — `IFillStrategy` interface, `FillContext`, `FillStrategyRegistry` (auto-discovers strategies via reflection, supports plugin DLLs), `FillHelpers`. Built-in strategies: `LinearFillStrategy`, `PairsFillStrategy`, `RectBestFitStrategy`, `ExtentsFillStrategy`.

Engines/Build-Engines.ps1

@@ -0,0 +1,40 @@
+<#
+.SYNOPSIS
+    Builds every plugin engine under Engines/ and deploys it to the benchmark.
+
+.DESCRIPTION
+    OpenNest.Benchmark loads plugin engines from an Engines/ folder next to its own
+    build output. This builds the benchmark plus each Engines/OpenNest.Engine.*/ project
+    (test subprojects are skipped) and copies each engine DLL into that folder.
+
+.EXAMPLE
+    ./Engines/Build-Engines.ps1
+    ./Engines/Build-Engines.ps1 -Engines Opus55,Terra -Configuration Debug
+#>
+param(
+    [string]$Configuration = 'Release',
+    # Engine names without the OpenNest.Engine. prefix; default is all of them.
+    [string[]]$Engines
+)
+
+$ErrorActionPreference = 'Stop'
+$repoRoot = Split-Path $PSScriptRoot -Parent
+
+dotnet build (Join-Path $repoRoot 'OpenNest.Benchmark/OpenNest.Benchmark.csproj') -c $Configuration
+if ($LASTEXITCODE -ne 0) { throw 'OpenNest.Benchmark build failed.' }
+
+$deployDir = Join-Path $repoRoot "OpenNest.Benchmark/bin/$Configuration/net8.0/Engines"
+New-Item -ItemType Directory -Force $deployDir | Out-Null
+
+$projects = Get-ChildItem $PSScriptRoot -Directory -Filter 'OpenNest.Engine.*' |
+    Where-Object { -not $Engines -or $Engines -contains $_.Name.Substring('OpenNest.Engine.'.Length) }
+
+foreach ($dir in $projects) {
+    $csproj = Join-Path $dir.FullName "$($dir.Name).csproj"
+    dotnet build $csproj -c $Configuration
+    if ($LASTEXITCODE -ne 0) { throw "$($dir.Name) build failed." }
+
+    $dll = Join-Path $dir.FullName "bin/$Configuration/net8.0/$($dir.Name).dll"
+    Copy-Item $dll $deployDir -Force
+    Write-Host "Deployed $($dir.Name) -> $deployDir"
+}

Engines/Directory.Build.props

@@ -0,0 +1,15 @@
+<Project>
+  <!--
+    Shared settings for out-of-solution nesting engine plugins. Each engine lives in
+    Engines/OpenNest.Engine.<Name>/ with an optional tests/ subproject; both import this.
+    AssemblyName and RootNamespace default to the project file name.
+  -->
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(MSBuildThisFileDirectory)../OpenNest.Engine/OpenNest.Engine.csproj" />
+  </ItemGroup>
+</Project>

Engines/Directory.Build.targets

@@ -0,0 +1,10 @@
+<Project>
+  <!--
+    Each engine's tests/ subproject sits inside the engine folder. This must live in a
+    .targets file: removals in Directory.Build.props run before the SDK adds its default
+    Compile glob, so they would have no effect.
+  -->
+  <ItemGroup>
+    <Compile Remove="tests/**/*.cs" />
+  </ItemGroup>
+</Project>

Engines/OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj

@@ -0,0 +1,6 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
+  <ItemGroup>
+    <InternalsVisibleTo Include="OpenNest.Engine.Opus55.Tests" />
+  </ItemGroup>
+</Project>

Engines/OpenNest.Engine.Opus55/README.md

@@ -61,8 +61,8 @@ Every placement decision (which part, which rotation, where, on which sheet) com
 ## Build / test
 
 ```bash
-dotnet build OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj -c Release
-dotnet test  OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
+dotnet test  Engines/OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln`, the same pattern as the
@@ -73,10 +73,12 @@ This project is intentionally **outside** `OpenNest.sln`, the same pattern as th
 ```bash
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Opus55/bin/Release/net8.0/OpenNest.Engine.Opus55.dll OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Opus55/bin/Release/net8.0/OpenNest.Engine.Opus55.dll OpenNest.Benchmark/bin/Release/net8.0/Engines/
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-manifest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Opus55`.
+
 The engine reports as `Opus55NestingEngine`.
 
 ## Known limitations

Engines/OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,8 +11,7 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Opus55.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
     <!-- The benchmark's NestValidator is the arbiter the engine is scored by. -->
-    <ProjectReference Include="../../OpenNest.Benchmark/OpenNest.Benchmark.csproj" />
+    <ProjectReference Include="../../../OpenNest.Benchmark/OpenNest.Benchmark.csproj" />
   </ItemGroup>
 </Project>

Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj

@@ -0,0 +1,3 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
+</Project>

Engines/OpenNest.Engine.Qwen/README.md

@@ -31,7 +31,7 @@ engines to start; it must not be the same algorithm re-derived through indirecti
 ## Build
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln` (same pattern as the
@@ -44,14 +44,16 @@ as part of the main solution.
 own build output:
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
 
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Qwen`.
+
 Your engine will show up in the report under its CLR type name (`QwenNestingEngine`),
 competing on equal footing against the built-in engines.

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

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,6 +11,5 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Qwen.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>

Engines/OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj

@@ -0,0 +1,3 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
+</Project>

Engines/OpenNest.Engine.Terra/README.md

@@ -31,7 +31,7 @@ engines to start; it must not be the same algorithm re-derived through indirecti
 ## Build
 
 ```bash
-dotnet build OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj
+dotnet build Engines/OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln` (same pattern as the
@@ -44,14 +44,16 @@ as part of the main solution.
 own build output:
 
 ```bash
-dotnet build OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj -c Release
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Terra/bin/Release/net8.0/OpenNest.Engine.Terra.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Terra/bin/Release/net8.0/OpenNest.Engine.Terra.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
 
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Terra`.
+
 Your engine will show up in the report under its CLR type name (`TerraNestingEngine`),
 competing on equal footing against the built-in engines.

Engines/OpenNest.Engine.Terra/tests/OpenNest.Engine.Terra.Tests.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,6 +11,5 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Terra.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>

OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj

@@ -1,14 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Opus55</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Opus55</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <InternalsVisibleTo Include="OpenNest.Engine.Opus55.Tests" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
-</Project>

OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj

@@ -1,13 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Qwen</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Qwen</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
-</Project>

OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj

@@ -1,13 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Terra</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Terra</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
-</Project>

README.md

@@ -265,6 +265,8 @@ An engine's layout is rejected (it places nothing and pays the penalty on every
 
 Custom competitor engines can be added by dropping a DLL that implements `INestingEngine` with a public parameterless constructor into the `Engines/` directory next to the benchmark executable; each one is registered under its own CLR type name. This jobs plug-in contract is also the supported extension point for new nesting engines. Plugins built for the removed single-plate inheritance API are not binary compatible and must be migrated to `INestingEngine`.
 
+In-repo plugin engines live in the top-level `Engines/` source folder, one `Engines/OpenNest.Engine.<Name>/` project each with an optional `tests/` subproject. They are outside `OpenNest.sln` and share their build settings and `OpenNest.Engine` reference through `Engines/Directory.Build.props`. `./Engines/Build-Engines.ps1` builds the benchmark and every engine, then copies the engine DLLs into the benchmark's runtime `Engines/` folder; pass `-Engines Opus55,Terra` to build a subset.
+
 ### Conservative bend endpoint repair (opt-in)
 
 Bend endpoint repair is disabled by default in the shared CAD importer.
@@ -314,6 +316,7 @@ OpenNest.sln
 ├── OpenNest.Core/              # Domain model, geometry, and CNC primitives
 ├── OpenNest.Engine/            # Nesting algorithms and whole-job contracts
 ├── OpenNest.Engine.Tests/      # Cross-platform whole-job contract tests (net8.0)
+├── Engines/                    # Out-of-solution plugin engines (OpenNest.Engine.<Name>/)
 ├── OpenNest.IO/                # File I/O — DXF import/export, nest file format
 ├── OpenNest.IO.Tests/          # Cross-platform CAD import and bend repair tests (net8.0)
 ├── OpenNest.Console/           # Command-line interface for batch nesting