Account for coat IOR in base roughening

As discussed on [Slack](https://academysoftwarefdn.slack.com/archives/C06365EAQMP/p1725421120304839?thread_ts=1724344444.282599&cid=C06365EAQMP), the coat roughening formula we suggest in the spec:


incorrectly ignores the IOR of the coat. For example if the coat IOR is 1, the coat is effectively not present so there should be no roughening effect.

Generally, I think we need to do some simulations to check the actual behavior, and come up with a good fitting formula, as the theory seems a bit hand wavy at present.. At least it should be understood that these formulas are just suggested approximations, but it's not good if they're totally wrong in certain regimes.

Start by reviewing the coat roughening formula in the specification and the linked Slack discussion. Run simulations across coat IOR values, especially IOR 1, and compare the results with the expected absence or presence of roughening. Done means the behavior is understood and the specification contains a fitting formula with its approximation limits documented.

Written by the indexing model from the issue text.