Function scatter_column 

#[doc(hidden)]
pub unsafe fn scatter_column(
    src: SEXP,
    present_idx: &[usize],
    n_rows: usize,
) -> SEXP

Scatter a typed column SEXP from a dense inner data frame into a new SEXP of length n_rows, placing NA/NULL at rows not in present_idx.

This is called by DataFrameRow-derived enum code to flatten struct-typed variant fields into prefixed columns of the parent data frame.

The output type mirrors the input:

• REALSXP → REALSXP (NA_real_ fill)
• INTSXP → INTSXP (NA_integer_ fill)
• LGLSXP → LGLSXP (NA_logical fill)
• STRSXP → STRSXP (NA_character_ fill)
• VECSXP → VECSXP (R_NilValue fill)
• anything else → VECSXP (R_NilValue fill)

Safety

Must be called on the R main thread. src must be a valid SEXP of length >= present_idx.len(). n_rows must equal the total row count of the parent data frame.