initialize

At the start of each replication, KSL's beforeReplication has already reset every TWResponse (including this aggregate and every observed source) to its initialValue. But the aggregate's initialValue is independent of its sources, so the aggregate restarts at 0 (or its declared value) even when sources start positive — and the first delta-tracking observation on a source's decrement then takes the aggregate negative, violating TWResponse's [0, ∞) domain.

This override re-syncs the aggregate's value to the sum of its sources' current values during initialize(). Assigning value = sum fires this aggregate's own observers, so the same re-sync propagates upward through any chain of AggregateTWResponses observing one another — order-independent because the chain self-corrects via successive observer firings.