Add NFS server mode for ATLAS metadata filesystem

[jayvenn21]

Summary

• add an nfs mode that serves the ATLAS metadata filesystem directly over NFS
• keep FUSE mounting available through the explicit fuse mode
• keep materialize mode available for writing the ATLAS tree to a real directory
• update README usage for FUSE, direct NFS serving, and materialized output

Validation

• cargo fmt --check
• cargo check
• cargo test
• Verified FUSE mode by mounting /tmp/atlas_mount, listing atlas, reading metadata.json, and unmounting successfully
• Verified NFS mode by starting the NFS server, mounting it with mount_nfs, listing atlas, and reading metadata.json
• Verified materialized mode by generating /tmp/atlas_export/atlas and reading metadata.json