Playground Shadow Query

Run authored SQL over the shadow DB and return the rows.

Backs the row previews in the data-mart and SQL-example editors: the structural checks say what shape the SQL has, this says what it does. Execution happens in an ephemeral in-memory DuckDB holding generated rows (plus the account's CSV fixtures), the same data the playground runs on. Customer data is never touched, and user SQL never reaches Annie's app DB.