Function run_build_index 

pub fn run_build_index(
    db_dir: &Path,
    rule: Option<&str>,
) -> Result<String, CliError>

Pump every outstanding vector-index build to completion, one slice per call, printing a line per slice.

rule narrows the report to one rule; the pump itself always advances every pending build, because they share one write lock and splitting them would only mean holding it more often.