feat(benchmark): castle comparison scene window + native-API free models in README

- CastleComparisonWindow (CoreAI/Benchmarks/Castle Comparison Scene...): scans Assets/Benchmark/<model>
  castle prefabs, model checkboxes + per-model castle picker (defaults to newest), builds one scene with
  a large 3D TextMesh label above each castle naming the model and its score. One castle per model.
- README: add the two opencode free models tested through the new opencode native API (hy3-free 86.9 full,
  deepseek-v4-flash-free partial), with a rate-limit caveat.

Author: NeoXider