Function derive_struct_dataframe 

fn derive_struct_dataframe(
    row_name: &Ident,
    input: &DeriveInput,
    data: &DataStruct,
    df_name: &Ident,
    attrs: &DataFrameAttrs,
) -> Result<TokenStream>

Generate DataFrameRow expansion for struct types.

Produces:

• A companion struct {Name}DataFrame with Vec<T> columns
• impl IntoDataFrame for {Name}DataFrame
• impl From<Vec<{Name}>> for {Name}DataFrame
• impl IntoIterator (for named structs without expansion)
• Associated methods: to_dataframe, from_dataframe, from_rows, from_rows_par
• A compile-time IntoList assertion (for non-expanded named structs)

Handles fixed-array expansion ([T; N]), pinned-width Vec expansion (Vec<T> + width), and auto-expand Vec (Vec<T> + expand).