dd600mb over my primary disk; I haven't done that in many years.
Adam thankfully pointed out that my data should be towards the end of the disk and that there were backup superblocks. TestDisk restored the partition table and dumped backup superblocks; then
e2fsck -y -b block-from-testdisk-later-than-the-clobbseems to have at least recovered my home directory.
(Ironically, I bumped the "back" button on my mouse right after I wrote the previous version of this post, clobbering the post. Nice JS there, LJ.)