Constant FFI_NO_EXPLICIT_TEMPERATURE 

pub const FFI_NO_EXPLICIT_TEMPERATURE: c_double = f64::NAN; // NaN_f64

Sentinel explicit_temperature_c value meaning “no explicit temperature override” for the ballistics_density_altitude_* exports below (MBA-1366) — same NaN-means-absent convention FFIBallisticInputs::latitude already uses. Any NaN bit pattern is accepted (checked via is_nan(), not equality), matching that precedent.