One-off for WeaponMechanics/WeaponMechanics#246
(JuggernautStyle: PAY FEATURE checkbox. Arrow at muzzle 80, oak Through
slows to 40 → 50% damage.)
WM already multiplies motion in Through.handleThrough. DamageDropoff is
distance, not current speed. This pack: base * clamp(current/muzzle, 0, 1).