Function build 

pub fn build(
    recs: &[Rec],
    index: &Index,
    mode: InjectMode,
    strength: Strength,
    char_budget: usize,
) -> (String, Vec<String>)

Build the injection text for recs and return it alongside the ids actually injected (after the char budget is applied). strength must already be resolved (not Strength::Auto); Auto is treated as Soft.