chore: move plugin engines to the OpenNest-Engines repo

Engines now live in https://git.thecozycat.net/aj/OpenNest-Engines (history
carried over) so they can be published independently, and so a copy of
OpenNest handed to a model for an engine-building run contains no
competing engines. Engines still load at runtime from an Engines/ folder
next to the app/benchmark output; nothing in the solution referenced them.

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

CLAUDE.md

@@ -47,7 +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.
+- **Plugin engines**: independent `INestingEngine` plugins are class libraries that reference `OpenNest.Engine` and are built outside `OpenNest.sln`. The desktop app and `OpenNest.Benchmark` load them from an `Engines/` folder next to their build output (e.g. `OpenNest.Benchmark/bin/<Config>/net8.0/Engines/`). Do not add engine projects to this repo.
 - **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`.