Add a WrenAI MDL converter

Hi all 👋

I'd like to gauge interest in adding a converter between Ossie and **WrenAI's MDL**
(Modeling Definition Language) under `converters/`.

[WrenAI](https://github.com/Canner/WrenAI) is an open-source semantic layer / GenBI project.
Its MDL is a JSON manifest describing a semantic layer as models (tables/views with typed
columns), relationships, and cubes (pre-aggregated measures) under a catalog/schema
namespace — conceptually close to Ossie's datasets / fields / relationships / metrics, which
makes it a natural fit for a hub-and-spoke converter.

I'm interested in contributing a bidirectional converter (Ossie ↔ MDL) following the pattern
of the existing converters, and I'd be happy to follow the "Contributing a New Converter"
steps (vendor entry, both directions, tests off the TPC-DS example, a README). I've started
prototyping and would open a PR if this is welcome.

Before I go further: **would a WrenAI MDL converter be welcome in this repo?** And is there
anything you'd want me to keep in mind on approach or scope?

Thanks!

Start in converters/ and review the existing converter implementations and the “Contributing a New Converter” steps. Confirm the vendor entry, bidirectional scope, TPC-DS-based tests, and README requirements before defining what done means for the WrenAI MDL integration.

Written by the indexing model from the issue text.