A half-baked idea related to ssh format proposal, needs thinking and verification

              v-----the part that is hashed is §3 struct----v
checksum := H("stuff" + "namespace with shard hint" + H(data))
sig := Sign(sk, checksum)

sig over checksum is what ssh-keygen -Y produces given sk, namespace, data?

Submit with same formats as before:
- vk
- shard hint
- H(data), "easily produced with sha256sum"
- sig, "easily produced with ssh-keygen -Y and re-encoded to just be hex sig bytes"
- domain hint

Log computes checksum based on the above to verify signature
Log does the normal "ok shard hint, ok domain hint checks"
Log then accepts and creates leaf

Leaf
- checksum
- sig
- vk

shard_hint is not needed in leaf now, could be where data is
Monitor can verify signature if they know vk as before