Function run_expr_checks 

pub fn run_expr_checks(
    df: &DataFrame,
    raw_df: &DataFrame,
    required_cols: &[String],
    columns: &[ColumnConfig],
    track_cast: bool,
) -> FloeResult<ExprCheckResult>

Runs not_null and cast_mismatch checks using Polars columnar operations.

not_null checks are evaluated via a single lazy expression pass on df. cast_mismatch checks are computed directly from BooleanChunked masks derived from the raw and typed DataFrames, avoiding the need to combine them into one DataFrame.

track_cast should only be true when cast errors are known to exist (from a prior columnar count pass), so the cast path is skipped entirely on the happy path.