Function account_incoming_surbs 

fn account_incoming_surbs(
    estimator: &AtomicSurbFlowEstimator,
    saved: u64,
    evicted: u64,
)

Books an incoming packet’s SURBs into the Exit’s flow estimator.

saved SURBs entered the store and evicted older ones were dropped to make room for them. Both are recorded so that produced - consumed - evicted tracks what the store actually holds: without the eviction term the estimate stays inflated by every SURB the full store ever discarded, long after they are gone – which leaves the keep-alive level report and the PIX fill reserve both reading a level that never falls. Split out from the receive closure so the accounting can be unit-tested on its own.