Function rereplicate 

pub fn rereplicate(
    reader: &mut dyn BufRead,
    writer: &mut dyn Write,
    sizeout: bool,
    fasta_width: usize,
) -> Result<(u64, u64, u64)>

Core rereplicate logic.

Reads FASTA from reader and writes each record expanded abundance times to writer. Returns (amplicons, reads, missing_abundance_count).

sizeout appends ;size=1 to each emitted copy. fasta_width sets the sequence line wrap column (0 = no wrap).

Records without a ;size=N annotation are treated as abundance 1 and counted in missing_abundance_count.