EtfPerSymbolFlowPeriod(EtfPerSymbolFlowPeriod)
One filing-cadence period flow for one ETF.A period is bounded by two snapshot dates where the SEC-derived
(aum_usd, nav) pair CHANGED. from_date is the last snapshot carrying
the OLD pair; to_date is the first snapshot carrying the NEW pair.
Consecutive snapshots with unchanged (aum_usd, nav) are a single plateau
and produce NO period entry (no zero-flow filler).
{
"from_date": "2026-04-30",
"to_date": "2026-05-31",
"from_nav": 0,
"to_nav": 0,
"delta_shares": 0,
"net_flow_dollar_estimate": 0
}