refactor(config): require explicit BZOD data directory

src/db/topology.rs

@@ -10,7 +10,7 @@
 //!     └── extensions/<extension>/<extension>.db
 //! ```
 //!
-//! `<data_dir>` is `Config.data_dir` (env `DATA_DIR`, default `./data`).
+//! `<data_dir>` is `Config.data_dir` (CLI `--data-dir`, env `NX9_BZOD_DATA_DIR`, or config file).
 //! It is not renamed to `database/`. Production Docker, CasaOS, and
 //! `deploy.sh` bind this physical root.