I finally got around to fixing Windows’ bull-shit. Last night, I shut down my computer to save on energy, so this morning I took advantage of the fact that I would have to boot up anyways. I plugged in my thumb drive that I prepared the other day, and loaded up System Rescue CD. I used it to mount and inspect my ESP, which turned out to be perfectly fine!
Rather than screwing up my ESP, Windows bulldozed the EFI boot variables. Still very annoying, but easier to recover from. All I had to do was use efibootmgr to re-register the grub bootloader, and reconfigure the boot order variable. A simple reboot, and I’m back in my Gentoo installation!