I noticed in the kernel logs the following message:
[ 16.908469] EDAC amd64: Node 0: DRAM ECC disabled.
[ 16.908473] EDAC amd64: ECC disabled in the BIOS or no ECC capability, module will not load.
Either enable ECC checking or force module loading by setting 'ecc_enable_override'.
(Note that use of the override may cause unknown side effects.)
This means the error-correcting RAM I have isn't actually being used. My buddy Pluvius helped me look into this issue and we believe the ECC fetcher was removed in a recent BIOS update. They had some clever but misleading verbiage.
Support for ECC Un-buffered DIMM 1Rx8/2Rx8 memory modules (operate in non-ECC mode)
So you can buy ECC RAM and it will work—it just won't have any error correction. This is more than irritating. Pretty much all forms talking about running a ZFS file system strongly recommend using ECC RAM—which I did. However it isn't actually working.