Function create_incremental_inferred_fill 

pub fn create_incremental_inferred_fill(
    order: &OrderAny,
    report: &OrderStatusReport,
    account_id: &AccountId,
    instrument: &InstrumentAny,
    ts_now: UnixNanos,
) -> Option<OrderEventAny>

Creates an inferred fill for the quantity difference between order and report.

This handles incremental fills where the order already has some filled quantity.