Rama asteroid field collapse calculation

Assuming a mostly base silicate composition (as opposed to the numerous amounts of metals that form the core layers of a planet), the energy required to melt silicon from the background temperature of space to which it phases into a molten state is roughly 2.65 MJ/kg, at a density of roughly 2330 kg/m^3 the energy required to reduce a single cubic meter of rock to the latent point of fusion equals 6,174.5 MJ. The composition and scale of the planet will naturally change as the planet cools, but at 20,000 km reducing a volume of asteroidal rock would require 4.18 x 1021 m^3 of molten material - the total energy expenditure of such an effort would be in excess of 2.58 x 10^31 J (6.1 zettatons).

This energy expenditure is only for heating the material by the way and not making mention of the subsequent cooling of an entire planet over less than 10 millennia. And remember, this creation of planets was basically an industry in the Ecumene, meaning that the Forerunners expended such amounts of energy as part of their day to day operations.