I have put up some cement board for a new shower. Some photos he
https://ibb.co/album/k68k5a

The cement board is pretty straight and vertical. The rest of the
bathroom is not!

I am wondering how best to finish off the end of the cement board, next
to the door architrave? I could just squirt some expanding foam into the
gap, but that still leaves the edge of the cement board exposed.

One possibility is to rout a big groove in a length of timber and slot
that over the end. I was thinking of tiling it first, then the wood can
go over the edge of the whole lot.

Any suggestions most welcome, please.