Function build_output_idx_from 

fn build_output_idx_from(
    fs: &FileStore,
    block_id: &str,
    output_size: u64,
    scan_start: u64,
    seed: Vec<u8>,
    seed_count: u64,
    for_gen: Option<String>,
    append_to: Option<i64>,
) -> Option<u64>

Shared scanner behind both entry points.

scan_start MUST be the byte offset of a line start (0, or an offset read back out of the index). seed is the raw offset bytes for the lines before it and seed_count how many — they are copied through untouched, so the blank/CRLF rules only ever get applied by the one loop below and the two paths can’t drift apart.