Function populate_behavior 

pub fn populate_behavior(
    deltas: &mut BTreeMap<String, RuleDelta>,
    before: &[DecisionLine],
    after: &[DecisionLine],
) -> FlipCounter

Walk paired before/after decision lists, fill in fires_before / fires_after, build the global flip counter, attribute each flip to the rule(s) that materially changed under the after-state, and return the global flip counter.

Pairing is by index, mirroring the Python prototype. If the two runs produced different counts (which can happen if a shieldset change causes evaluation errors on some lines), we pair as many as we have and emit a stderr warning.