Function labelled_for 

fn labelled_for(meta: &HashMap<String, Value>, output_gen: Option<&str>) -> bool

Whether an index with metadata meta may describe output of generation output_gen, apart from its covered size.

A size match alone is not proof: output replaced or restored with content that happens to reach the old covered size would have its lines read at another file’s offsets. When output has a valid generation the index must be labelled with it; any mismatch — unlabelled (older build), or built for another generation — means rebuild, which is always safe. An uncounted output (legacy or written by an older build) keeps the covered-size rule alone, as before.