Add hybrid mnesia_rocksdb backup and restore

Provide mnesia_rocksdb:backup/1,2 and restore/1,2 that pair a Mnesia
checkpoint (schema and non-RocksDB tables) with RocksDB backup engines
for open admin and standalone databases.

Restore stages RocksDB into a sibling directory and redirects mountpoints
while Mnesia fallback purges the real directory, then relocates the data
and optionally restarts. Raise the minimum OTP version to 27.

README.md

@@ -43,7 +43,7 @@ is provided.
 
 * sext (included as dependency)
 
-* Erlang/OTP 21.0 or newer (https://github.com/erlang/otp)
+* Erlang/OTP 27.0 or newer (https://github.com/erlang/otp)
 
 
 
@@ -115,6 +115,65 @@ source code in `mnesia_rocksdb_tuning.erl` and `mnesia_rocksdb_params.erl`.
 Contributions are welcome.
 
 
+#### <a name="Backup">Backup</a> ####
+
+`mnesia_rocksdb:backup(Dir)` creates a **local hybrid backup** of Mnesia
+metadata/tables and RocksDB data. Unlike `mnesia:backup/1`, RocksDB-backed
+tables are not dumped record-by-record through Mnesia; they are captured with
+the [RocksDB backup engine](https://rocksdb.hexdocs.pm/1.9.0/how_to_backup_rocksdb.html).
+
+Layout of `Dir` after a successful backup:
+
+```
+Dir/
+  MANIFEST                 % restore metadata (Erlang term)
+  mnesia.BUP               % schema + non-RocksDB tables
+  rocksdb/<alias>/admin/   % backup engine dir for each alias admin DB
+  rocksdb/standalone/...   % one dir per open standalone RocksDB table
+```
+
+Algorithm (local node only):
+
+1. Activate a Mnesia checkpoint on **schema and non-RocksDB tables only**
+   (so large RocksDB tables do not get expensive checkpoint retainers).
+2. For each open RocksDB instance (admin DB per alias, and standalone tables),
+   run `rocksdb:create_new_backup/2`.
+3. Dump the checkpoint with `mnesia:backup_checkpoint/3`, excluding RocksDB
+   table data. Schema still describes RocksDB tables.
+4. Write `MANIFEST`.
+
+Options map (second argument to `backup/2`):
+
+* `{verify, true}` — call `rocksdb:verify_backup/2` after each RocksDB backup
+* `{mnesia_backup_module, Mod}` — override the Mnesia backup media module
+
+#### Restore
+
+`mnesia_rocksdb:restore(Dir)` restores a hybrid backup offline (local node).
+
+Mnesia applies fallbacks by **purging** the Mnesia directory. RocksDB data is
+therefore restored first into a **sibling staging directory** and mountpoints
+are redirected until the fallback has finished:
+
+1. Stop Mnesia (closes RocksDB handles).
+2. Restore each RocksDB DB into
+   `"<basename(mnesia_dir)>.rocksdb_restore/"` (sibling of the Mnesia dir).
+3. Set application env `mnesia_rocksdb.restore_mountpoint_root` so
+   `data_mountpoint/1` opens DBs from staging.
+4. Install `Dir/mnesia.BUP` and start Mnesia (fallback purges the real Mnesia
+   dir; RocksDB tables load from staging with full data). During this window,
+   `delete_table` does not destroy staged RocksDB files.
+5. Stop Mnesia, clear the redirect, move staged `*.extrdb` directories into the
+   real Mnesia directory.
+6. Optionally start Mnesia (`#{start => true}`).
+
+Common options for `restore/2`:
+
+* `{stop, true|false}` — stop Mnesia before restore (default `true`)
+* `{start, true|false}` — start Mnesia after finalize (default `false`)
+* `{mnesia_dir, Dir}` — override target Mnesia directory
+* `{skip_rocksdb, true}` / `{skip_mnesia, true}` — partial restore
+
 #### <a name="Caveats">Caveats</a> ####
 
 Avoid placing `bag` tables in RocksDB. Although they work, each write