Function fold_net_position 

pub fn fold_net_position(legs: &[(Decimal, Decimal, u64)]) -> (Decimal, Decimal)

Replays position legs onto a hypothetical NETTING position in ts_opened order, returning (net_signed_qty, net_avg_px_open).

Each leg is (signed_qty, avg_px_open, ts_opened_ns). Rules follow Position::apply:

• Same-side legs produce a quantity-weighted average open price.
• Opposite-side legs partial-close at the existing average.
• A leg that crosses zero makes the residual take that leg’s price.

Zero-quantity legs are skipped. Sort is stable on ts_opened; the caller orders ties (e.g. by position_id).