build: stand alone from the OpenNest repo

Engines are moving out of OpenNest so they can be published on their own
and so OpenNest checkouts handed to a model for an engine-building run no
longer contain the competing engines. History for Engines/ (and the
pre-move root-level engine folders) was carried over with git filter-repo.

Engines now build against a sibling OpenNest clone via $(OpenNestRoot),
overridable with /p:OpenNestRoot, with a clear error when it is missing.

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

README.md

@@ -0,0 +1,39 @@
+# OpenNest Engines
+
+Independent nesting engine plugins for [OpenNest](https://git.thecozycat.net/aj/OpenNest).
+Each engine is a standalone class library that OpenNest discovers at runtime via
+`NestingEngineRegistry.LoadPlugins()` — drop the DLL into an `Engines/` folder next to
+the OpenNest app or `OpenNest.Benchmark` build output.
+
+| Engine | Approach |
+|--------|----------|
+| [Astra](OpenNest.Engine.Astra/) | Contact-based placement |
+| [Opus55](OpenNest.Engine.Opus55/) | Frontier-advance no-fit-polygon packing |
+| [Qwen](OpenNest.Engine.Qwen/) | Scaffold — not yet implemented |
+
+## Building
+
+Engines compile against an OpenNest checkout, expected as a sibling clone:
+
+```
+src/
+  OpenNest/           git clone https://git.thecozycat.net/aj/OpenNest.git
+  OpenNest-Engines/   this repo
+```
+
+Use a different location with `/p:OpenNestRoot=<path>` (or the `OpenNestRoot`
+environment variable).
+
+```powershell
+./Build-Engines.ps1                    # build all engines + deploy to the benchmark
+./Build-Engines.ps1 -Engines Opus55    # just one
+dotnet test OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
+```
+
+Each engine's README covers its algorithm and benchmark results.
+
+## Layout
+
+Each engine lives in `OpenNest.Engine.<Name>/` with an optional `tests/` subproject.
+`Directory.Build.props` supplies the target framework and the `OpenNest.Engine`
+reference, so an engine's `.csproj` can be nearly empty.