So after taking apart and re-assembling the airbox and re-cleaning the MAF (just to be safe!) it appears the idle is back to normal, for the most part (by normal, I mean no stalls and back within a nice RPM range).
Unfortunately, a few hours later I got a CEL. I say "unfortunately", but it's actually kinda what I was hoping would happened. Pulled a code P0303.
I think I'm gonna change out the valve cover gaskets and spark plug gaskets for starters. Then replace the spark plug wires. The reason I'm tackling those first is because I noticed a little oil in the spark plug boot. Not a ton, but enough that I figure I should tackle it now before it becomes a problem. Do you think I should change the plugs as well? They're only a few weeks old. They're not too expensive so it wouldn't be a problem to change them, so I'd rather change them now if need be vs having to go back in after the fact to swap them out.
If none of the above work, I'll look into the coil pack, which I'm hoping is not the issue. With 180k miles on the Legacy, the gaskets and wires are probably due for a change anyway, so it won't be the end of the world if they don't work.