Function resolve_projectile_shape 

pub fn resolve_projectile_shape(
    bullet_model: Option<&str>,
    caliber: f64,
    weight_grains: f64,
    g_model: &str,
) -> ProjectileShape

Resolve the projectile shape for transonic corrections (MBA-949). A user-supplied bullet_model name (boat/bt, round/rn, flat/fb) takes precedence; otherwise fall back to the caliber/weight/ drag-model heuristic below. This name-parsing previously lived in cli_api and fast_trajectory but NOT derivatives, so named shapes were silently ignored on the integrate_trajectory path. Sharing one helper makes all three solver families resolve the same shape for the same inputs.