Function core_power_multiplier

pub fn core_power_multiplier(product: i64) -> f64

BAL-010: the signed combat multiplier for the product P of the two core levels — one symmetric continuous formula replacing the superseded bucket table. M_power = 1 + min(P/5, sqrt(P) − 0.2); the P/5 arm rules the early products, the sqrt arm takes over from P ≈ 5 and keeps late levels worth buying without exploding. P ≤ 0 (a character without cores) is exactly ×1.0, which is what keeps acceptance #14 (“no cores plays as before”) true by construction.