Function run_trends 

pub fn run_trends(
    db: &FactsDb,
    opts: &Options,
    paths: &[String],
) -> Result<Vec<TrendPoint>>

Build a per-(month, path) trend series restricted to paths for the trends multi-line widget (W9). The score per (month, path) is the count of revisions that touched the path during the month. Empty paths returns an empty Vec — the widget renders nothing.