nfs4.1: namespace + fattr4 attributes (PUTROOTFH/LOOKUP/GETATTR/ACCESS/SECINFO)

[hartsock]

Goal

The read-only namespace + attribute model: pseudo-fs root, filehandle ops, name
resolution, and the fattr4 bitmap attribute mapping onto the existing
PassthroughFs/std::fs metadata.

Scope

• PUTROOTFH (v4 pseudo-root = export root), PUTFH, GETFH, SAVEFH,
  RESTOREFH.
• LOOKUP, LOOKUPP, ACCESS.
• GETATTR/SETATTR over bitmap4 — map mandatory + recommended attrs
  (type, change, size, fsid, fileid, mode, owner/owner_group, time_*,
  maxread/maxwrite) onto std::fs::Metadata; reuse the (dev,ino) fileid and
  the F5 ownership/chown semantics for owner/mode SETATTR.
• SECINFO/SECINFO_NO_NAME advertising AUTH_SYS.
• Filehandle compatibility with the v3 handle codec where possible.

Dependencies

#54, #55.

Acceptance

• ls -la, stat, path traversal over a vers=4.1 mount return correct attributes;
  owner/group reflect the AUTH_SYS caller (consistent with F5).