Close Unity Catalog and governance parity gaps for embedded deployments

[ethan-tyler]

Summary

Unity Catalog facade coverage and governance or grant parity are not complete enough yet for embedded, product-owned control plane routes.

Severity: high

Downstream evidence

We still have to:

• leave some UC routes as placeholders because the compatibility surface is not complete enough yet
• keep product-level policy authoring outside Arco and project only a subset of grants inward

Requested outcome

• Document a UC endpoint coverage matrix
• Define the minimum embedded governance and grant model
• Add compatibility tests for common engine-facing catalog and grant flows
• Track the remaining parity gaps explicitly in the repo

Why upstream

Embedders need a clear line between supported behavior and product-owned projection logic. Without that, downstream services either ship placeholders or invent their own governance story.

[ethan-tyler]

Concrete downstream evidence from Daxis:

• Our architecture docs still keep product-level policy authoring outside Arco because governance parity is not complete enough yet.
• The current embedded catalog service still leaves /api/2.1/unity-catalog/catalogs as a 501 placeholder in crates/daxis-catalog/src/handlers.rs.

That is a strong sign embedders still need a clearer upstream UC coverage line and grant/governance story.