I'll give the whole story if anyone wants it
Okay, so here's the recap:
I woke up this morning and decided my main drive (just a 500GB SSD) was too full, at about 85%, so I decided to do something about that. I go through the usual: pacman -Sc, paccache -rk0, and pacman -Qqtd | pacman -Rns - (which I've aliased to "orphankiller" because that's too much typing for me). None of that did anything though, as I'm usually pretty up on this, and I expected it, so my next step was to find other ways of deleting unnecessary files floating around, and that meant a trip to the usually very helpful Arch wiki.
On the page "pacman Tips and Tricks", I find 1.7: Detecting More Unneeded Packages. "Perfect!" I thought, "That's exactly what I'm looking for!" I enthusiastically type in the command pacman -Qqd | pacman -Rns -, and then quickly go check how much space I just saved. Nada. Or at least not enough to move the percentage point. "Oh well, keep looking," I think and I go back to Firefox to click some more links in hopes that one of them will be the space saving ultra-script that I need. The first one I click, I get an error from my trusty browser, I don't remember exactly what it was but it was something about not being able to verify the page. "Weird, let's try another one." Nope, same thing.
Well, being that I had just deleted something, I figured I should go see what exactly it was that I did. It was a good thing I'd left the terminal window open, because after just a few scrolls I saw it: ca_certificates, which Firefox absolutely needs. "Great, I'll just reinstall." Nope! I just deleted my pacman cache, and pacman also needs those certificates to download from the Arch repo's mirrors! "Fantastic," I grumbled while I tried to think of how I could get this pesky package back on my machine.
Then it occurred to me: I've been keeping up with my btrfs snapshots (for once, lol)! I can just backup to yesterday and forget this whole mess! So I bring up Timeshift, and we're on our way back to a functioning system! Or so I thought. See, I don't have a separate /home partition, but I do have a separate @home subvolume, so when Timeshift asked me if I wanted to restore that too, I clicked the check mark. Only thing is, I don't think I actually have a separate @home subvolume, which brings us to the error in the meme. /home wouldn't mount, and that meant I was borked.
Fortunately, our story has a happy ending! I DDG'd the error on my phone, and found a post from like seven years ago, about someone who had this same set of circumstances, and the one reply was my fix: just go into /etc/fstab and delete the "subvolid" part of whatever partition that's giving you grief. Did that, reboot, and we're finally fixed! And now, forevermore, I shall check what I'm deleting before I hit the enter button!
The post-script is bittersweet though, because after all this trouble, and then the rest of the afternoon working on the original problem, I am down to... 81%. Oh well.