DATA-N4.8.1 additive NAV price-proxy block.Surfaced ONLY when the snapshot has no exchange price (price is null) but a
SEC-derived NAV is available. The NAV is offered as an explicitly-stale
reference value - never as a current quote. The price field itself
stays null, so a client keying on price sees absence, not a fabricated
number. Absent when price is present or when nav is also null.
{
"value": 0,
"basis": "nav",
"as_of": "string",
"method": "string",
"note": "string"
}