feat(nuget): transitive dependency resolution + editor-only assemblies

[klumhru]

Summary

Adds two opt-in capabilities to the nuget package type:

• nugetResolveDependencies — walks the package's .nuspec dependency graph, downloads each transitive dependency, and extracts its DLLs into the same folder. Framework matching prefers an exact target, falling back to the nearest-compatible lower netstandard version; versions resolve to the range lower bound; package ids dedupe across diamond graphs.
• editorOnly — routes extracted assemblies (and resolved deps) into an Editor/ special folder instead of Plugins/, for editor-only libs like Roslyn.

Framework/runtime meta-packages and Unity-provided System.* facades (System.Memory, System.Buffers, System.Runtime.CompilerServices.Unsafe, System.Numerics.Vectors, System.Threading.Tasks.Extensions, System.ValueTuple) are skipped to avoid duplicate-assembly conflicts. Meta-packages with no lib/ assemblies are tolerated — their own dependencies are still resolved.

Changes

• internal/packager/nuspec.go (new) — .nuspec parsing, framework normalization, version-range parsing, dependency-group selection, skip list.
• internal/packager/nuget.go — recursive resolveNuGetDeps, best-framework lib/ selection, Editor/ vs Plugins/ routing.
• internal/config/config.go — nugetResolveDependencies + editorOnly fields.
• Docs: README.md, CLAUDE.md.

Testing

• Unit tests: framework normalization, version-range resolution, skip list, dependency-group selection, nuspec parsing, best-framework selection.
• Integration tests: TestIntegration_NuGet_ResolveDependencies (Grpc.Core, asserts facades skipped), TestIntegration_NuGet_EditorOnly (Newtonsoft.Json → Editor/).
• Manually verified against Microsoft.CodeAnalysis.CSharp 4.8.0: full Roslyn tree resolved, facades dropped, assemblies placed under Editor/.

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