Function decide 

pub fn decide(
    snapshot: &Snapshot,
    request: &ReplayRequest,
    rate_counter: &mut SimulatedCounter,
) -> ReplayDecision

The pure decision function (DW-102). Re-runs the gateway’s decision path — route match, authz, simulated rate limit, transform resolution, upstream pick — against snapshot for request, with NO I/O and NO side effects on live gateway state. The rate_counter is the ONLY mutable state touched, and it is a simulated counter the caller owns (not the live limiter).

Returns a ReplayDecision describing what the live proxy WOULD have decided at each stage. Stages after a miss (no route, authz deny, rate-limit deny) are still reported when they can be computed without side effects — an operator diffing two configs wants to see “authz now denies” even when the live proxy would have stopped there, so the diff surfaces the full picture.