fix(nuget): narrow legacy project.json recognition for GitLab verification

[mstykow]

Summary

• verify gitlabhq/gitlabhq with the maintained compare-outputs workflow in the context of PR 826's Ruby changes, and record the reviewed benchmark snapshot in docs/BENCHMARKS.md
• stop treating generic GitLab export project.json fixtures as legacy NuGet manifests by requiring legacy NuGet shape keys before parser extraction or fallback rows
• align the NuGet improvement docs and generated supported-formats matrix with the narrowed legacy project.json behavior

Issues

• Covers: GitLab Ruby-lane verification for the PR 826 static-resolution work
• Closes:

Scope and exclusions

• Included:
  • src/parsers/nuget/project_json.rs
  • focused regression coverage in src/parsers/nuget/nuget_test.rs
  • docs/BENCHMARKS.md and docs/benchmarks/scan-duration-vs-files.svg
  • docs/improvements/nuget-parser.md and generated docs/SUPPORTED_FORMATS.md
• Explicit exclusions:
  • no broader parser behavior changes outside legacy project.json recognition
  • no benchmark claims beyond the recorded gitlabhq/gitlabhq compare snapshot

Intentional differences from Python

• legacy NuGet project.json support now requires legacy-manifest shape rather than filename alone, which keeps generic GitLab export fixtures out of NuGet output while preserving fallback rows for malformed but still probable legacy NuGet manifests
• the GitLab benchmark explicitly validates the Ruby slice of PR 826: Provenant resolves real gem versions from local Ruby constants where ScanCode still leaves ::VERSION placeholders

Follow-up work

• Created or intentionally deferred:
  • reviewed compare artifacts:
    • .provenant/compare-runs/20260430T174411Z-gitlabhq-45077
    • .provenant/compare-runs/20260430T194138Z-gitlabhq-85785
  • no new goldens were required for this change; the focused parser and golden checks passed unchanged
  • validations run:
    • cargo test test_project_json_
    • cargo test --features golden-tests test_golden_project_json
    • cargo run --manifest-path xtask/Cargo.toml --bin generate-benchmark-chart
    • cargo run --manifest-path xtask/Cargo.toml --bin generate-supported-formats