//! Engine-agnostic plan executor trait.
use crateDataFrameBackend;
use crateSparkSessionBackend;
use crateEngineError;
use Value as JsonValue;
/// Executes a logical plan (JSON list of ops) and returns a DataFrame.
///
/// The generic type parameter `S` is a backend-specific [`SparkSessionBackend`] implementation
/// (e.g. the Polars-backed `SparkSession` in `robin-sparkless-polars`). Backends implement this
/// trait to expose plan execution in terms of engine-agnostic traits and [`EngineError`].