Function resolve_station_pressure 

pub fn resolve_station_pressure(
    pressure_hpa: f64,
    altitude_m: f64,
) -> Option<f64>

Resolve the station-pressure override for an air-density calculation.

Altitude and pressure are redundant inputs for density. The rule:

• An explicitly-supplied pressure is the authoritative STATION pressure (already altitude-reduced); it is returned as Some and used directly, so altitude is NOT double-counted.
• When pressure is left at the sea-level standard default (≈1013.25 hPa) while a real altitude is given, the caller meant “standard atmosphere at this altitude”: return None so calculate_atmosphere derives the station pressure from altitude (ICAO standard) instead of silently using sea-level density.

Without this, --altitude with the default pressure produced sea-level density (altitude had no effect on drag). The ±0.5 hPa tolerance covers the 29.92 inHg ≈ 1013.21 hPa conversion, and >1 m avoids triggering at sea level. (Mirrors the existing pressure != 29.92 “user override” sentinel used elsewhere in the CLI.)