Function weighted_pick_with_index 

pub fn weighted_pick_with_index<'a, T>(
    entries: &'a [(u32, T)],
    key: &str,
) -> (&'a T, usize)

DW-091: like weighted_pick but also returns the picked entry’s INDEX (0 = baseline, 1 = canary for a 2-version canary split). The auto-canary controller needs the index to record the outcome on the correct side. Same hash and slot arithmetic.