Function counted_line_count 

fn counted_line_count(
    fs: &FileStore,
    zone: &str,
    stream: String,
) -> Option<BlockfileLineCountResult>

A transcript stream’s line count from its counter, with the stream and generation it counts (Phase 5a-3). A file with no epoch yet gets one first — init_line_counter, one read of the file, once per epoch; call on the blocking pool. None when it can’t be counted (no such file, or mid-write by a build without transactions), and the caller falls back.