My Civic has been decapitated!
After fighting with a leaky headgasket for a while, I finally followed Daox's lead and got the head pulled off to deal with it. There's a little carbon buildup on the pistons. Hmm...
Cylinder 4 seems to some black soot between the cylinder and the coolant passages. Not shown, but the gasket shows some small signs of deterioration on all four cylinders. It was definitely time to replace.
I took the head to a machine shop and he verified there were no cracks...
but also verified I had multiple valves that were not sealing and the valve seals were letting oil past on several valves (confirming that burnt oil odor I thought I smelled on startup). No quick resurface here, the head is getting a full rebuild.
Hopefully this translates to a significant uptick in mpgs afterwards...