Move 43 root-level test files into feature-specific subdirectories
mirroring the main codebase structure: Geometry, Fill, BestFit, CutOffs,
CuttingStrategy, Engine, IO. Update namespaces to match folder paths.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Renamed OpenNest.Engine.Tests → OpenNest.Tests (directory, .csproj,
namespaces in all .cs files). Added OpenNest.IO project reference.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>