Support the dbt Semantic Layer as a query source

## Summary

Boards reach data through SQL today. A dbt project that defines metrics in the Semantic Layer has to restate those metrics as SQL in every board, which duplicates the definition and lets the two drift.

## Proposal

Let a board use a metric as the dbt project defines it, so the definition lives in one place and every board that uses it agrees. The authoring shape is not designed yet; it will follow the Semantic Layer's next release rather than the current API.

## Status

Planned, not yet scheduled. This issue is the place to follow progress and to add use cases.