Store POSIX file attributes (uid, gid, mode, mtime) as x-amz-meta-* headers on S3 objects so POSIX filesystems layered on Tigris can round-trip ownership and permissions instead of always reporting mode 0666 with no owner. The feature is opt-in and off by default: NewS3FS takes functional options, and without WithUnixMetadata the filesystem behaves exactly as before. Callers opt in by passing WithUnixMetadata(uid, gid, umask) and resolve any name strings to numeric IDs themselves. New unixmeta package implements Encode/Decode and PosixMode/GoFileMode conversion; the wire format is documented in docs/reference. Assisted-by: Claude Opus 4.7 via Claude Code Signed-off-by: Xe Iaso <me@xeiaso.net>