refactor(misconf): use Helm v4 SDK loaders instead of custom tar parsing

[nikpivkin]

Trivy currently parses Helm chart archives manually (parser_tar.go). Switching to the Helm v4 SDK loaders (LoadArchive, LoadDir) offloads chart version detection to the SDK — which is a prerequisite for Charts v3 support.

Scope

• Replace custom tar parsing in parser_tar.go with loader.LoadArchive
• Convert the resulting *chart.Chart into Trivy's mapfs.FS for downstream scanning
• Verify existing test coverage still passes

References

• HIP-0020: Charts v3 Enablement