fix(xdge): pass all merlint checks (0 issues)
- Extract pp_source/pp_path_with_source/pp_path_opt_with_source/pp_paths
to top-level to fix E005 on pp (76→34 lines)
- Extract resolve_dir_from_config/resolve_env_var/print_path/print_paths
to Cmd module level to fix E010 deep nesting in term (depth 5→3)
- Rename create→v, find_config_file→config_file, find_data_file→data_file,
find_file_in_dirs→file_in_dirs (E331/E332)
- Fix E410: add missing periods to all doc strings in xdge.mli
- Fix E105: replace catch-all with Eio.Io in test_xdge.ml
- Add test_xdge.ml/mli, test.ml, update dune for E605
- Add suite to test_paths for E600/E621