It seems like this should be obvious, but I'm not finding an answer. LGPL is a subset of GPL, IIRC, so can I use LGPL code in a GPL project?
LGPL-3.0 is implemented as the GPL-3.0 plus additional permissions under Section 7 of the GPL-3.0. “When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it.” I.e. you can ignore these extra permissions if you want to.
So you are free to use LGPL-3.0 code as if it were GPL-3.0 licensed (because it is).
As pointed out by Bart van Ingen Schenau in the comments, this is also explained in the GPL license family compatibility chart in the GPL FAQ.