Function fallback_bullet_length_m 

pub fn fallback_bullet_length_m(diameter_m: f64, mass_kg: f64) -> f64

Resolve a bullet length (meters) for a bullet whose length the shooter did not supply (MBA-1135).

Prefers the mass-based physical estimate (crate::stability::estimate_bullet_length_m), falling back to the historical 4.5-caliber heuristic only when mass is unavailable so the caller always gets a positive length. diameter_m and mass_kg are SI.