Function stamps_for 

pub(crate) fn stamps_for(
    read: &dyn Fn(i64, i64) -> Result<Option<Vec<u8>>, StoreError>,
    size: i64,
    line_offsets: &[u64],
) -> Result<Option<Vec<i64>>, StoreError>

Stamps for line_offsets (ascending byte offsets in output), read from a sidecar of size bytes through read(offset, len) — None from read means those bytes aren’t all stored (a writer mid-append), and so no stamps. Ok(None): no sidecar records near the range at all.