Function create_synthetic_venue_order_id 

pub fn create_synthetic_venue_order_id(
    fill: &FillSnapshot,
    instrument_id: InstrumentId,
) -> VenueOrderId

Create a synthetic VenueOrderId for a derived fill.

The suffix is hashed from the fill fields and instrument so distinct fills never collide, and the same logical fill always yields the same VenueOrderId across restarts.

Format: S-{hex_timestamp}-{hash_suffix}