We were using an alphabet with a duplicate character on purpose.
Go 1.21 was perfectly fine with that, but 1.22 started noticing:
panic: encoding alphabet includes duplicate symbols
I can't fault the new sanity check, because it makes sense in general.
What we are doing here is slightly bizarre, because we don't care
about decoding the name hashes at all.
Appease the sanity check by replacing dashes with duplicate characters
as a follow-up step. While here, use "a" rather than "z",
which is more common and less likely to be noticeable.