Function build_index_on 

pub fn build_index_on(
    db: &mut GraphDb<RealFs>,
    rule: Option<&str>,
) -> Result<String, CliError>

run_build_index against an already-open handle.

Terminates: every pump advances each pending build’s cursor by at least one node (the slice size is never zero), so the outstanding list empties.

Exposed for tests that need a build this handle itself deferred. Whether a build is outstanding is decided when the rule is created, so a test using a reduced slice size has to keep the handle that created the rule; reopening replays CreateRule at the production slice size. A build a snapshot cut short needs no such care — the reopen recognises it, which is what run_build_index relies on.