Custom Spawn Rates

Per-nest control over enemy spawn composition. Adds a startup setting for every unit-spawner prototype (biter-spawner, spitter-spawner, egg-rafts, plus any added by mods such as Colossal Enemies). Each setting takes entries like 'medium-biter 0.2=0.00,0.6=0.40' to add or override a unit's evolution/rate table, or '-small-biter' to remove a unit from the nest entirely. Rates are relative shares of that nest's spawns, normalised across its units at every evolution level — same semantics as vanilla result_units.