Immutable DB: verify that no file handles are leaked

Make sure that no file handles are leaked by the Immutable DB. We can probably use the `Error` `HasFS` for this.

Start by locating the Immutable DB implementation and the Error HasFS handling mentioned in the issue. Add or run checks that exercise opening and closing the database, and consider the work done when those checks demonstrate that no file handles remain leaked.

Written by the indexing model from the issue text.