fix(symlinks,rename,replication): fix external symlink encoding, cyclic...

fix(symlinks,rename,replication): fix external symlink encoding, cyclic rename, and degraded read failover

- Tag cross-mount external symlinks with '/__gkfs_ext__' so internal symlinks preserve relative paths to mountdir and readlink returns proper paths.
- Enforce max symlink depth with ELOOP in get_metadata and gkfs_open_impl.
- Fix cyclic rename handling in gkfs_rename to restore stubs and clean up source files.
- Guard forward_rename against creating nodes with empty paths when clearing stubs.
- Allow inline data writes at offset 0 during cyclic rename restore.
- Trigger replica retry on degraded chunk reads when a server reports 0 bytes for non-empty chunk request.
- Ensure all code passes check_format.sh.