Add unit tests for the builder

The project is working but have a very big issue: there is no tests. Everything is tested by hand...

The goal here is to add unit tests to the builder

Start by locating the builder implementation and reading how ShieldDB constructs its embedded database and JSON-backed data. Add unit tests for the builder's observable behavior and run the project's existing test command to verify them. Done means the builder is covered by repeatable automated tests rather than manual checks.

Written by the indexing model from the issue text.