Every account carries settled, netFlowRate, and lastSettled. The realtime
balance is linear accrual the chain computes on read:
realtimeBalanceOf(a) = settled(a) + netFlowRate(a) * (now - lastSettled(a))
This is Superfluid's Constant Flow Agreement condensed into one contract over the
native token. A 4-hour solvency buffer is locked at createFlow so a stream can
never overdraw.