Spark transformer that expands all DSL entities (reads, forms, calculations, explicit derives, multi_selects, toggles) into field specs at compile time.

The transformer persists pure-data specs (no closures) via Transformer.persist/3. At runtime, build_fields/1 converts these specs into Lavash.Derived.Field structs with compute closures — called once per module by Dsl.Graph.compiled_graph and cached in persistent_term.