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.Terra/README.md

@@ -0,0 +1,57 @@
+# OpenNest.Engine.Terra
+
+An independent `INestingEngine` implementation — **not** a wrapper, ensemble, or
+selector over OpenNest's built-in engines (`StockLadderNestingEngine`,
+`FixedStrategyNestingEngine` "Default"/"Strip"/"Vertical Remnant"/"Horizontal Remnant`,
+or anything reachable through `PlateNesterFactory`/`NestingEngineRegistry`).
+`Solve()` must never call, instantiate, or otherwise delegate a placement decision
+to one of those.
+
+## Allowed building blocks
+
+Low-level geometry/data-structure primitives are fair game — they are not nesting
+strategies:
+
+- `OpenNest.Core` geometry: `Polygon`, `Shape`, `BoundingBox`, `Vector`, `Box`,
+  `ConvexHull`, `ConvexDecomposition`, `RotatingCalipers`, `Collision` (overlap/spacing
+  checks), `NoFitPolygon`, `ShapeProfile`, `SpatialQuery`.
+- `OpenNest.Engine` support types if useful: `PartBoundary`, `RotationAnalysis`,
+  `AngleCandidateBuilder` — the *decision logic* using them must be your own (don't just
+  call `BestFitFinder`/`PairEvaluator`/`RotationSlideStrategy`, which are the existing
+  best-fit engine's internals).
+
+## What to fill in
+
+`TerraNestingEngine.cs` — implement `Solve()`. Pick and document an actual
+placement strategy (NFP-based sliding placement, skyline/shelf packer,
+simulated-annealing/genetic layout search, guillotine-cut packer,
+physics/gravity-settling, etc). It's fine to be simpler or worse than the built-in
+engines to start; it must not be the same algorithm re-derived through indirection.
+
+## Build
+
+```bash
+dotnet build OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj
+```
+
+This project is intentionally **outside** `OpenNest.sln` (same pattern as the
+`OpenNest.Engine.Aurora` plugin) — it's discovered at runtime as a plugin, not built
+as part of the main solution.
+
+## Try it out with the benchmark
+
+`OpenNest.Benchmark` auto-loads plugin engines from an `Engines/` folder next to its
+own build output:
+
+```bash
+dotnet build 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/
+
+dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
+```
+
+Your engine will show up in the report under its CLR type name (`TerraNestingEngine`),
+competing on equal footing against the built-in engines.