Move ShardFor/Kind() out of the understanding layer

ShardFor (and the CheckpointID.Kind() method it used) had no production caller in this PR — their only consumer is the git-refs store's RefName, which lands with the ref-store work — so their doc comments had to forward-reference a per-checkpoint ref store that isn't here. Sharding is a storage concern, not part of recognizing/validating an ID, so move both (and their tests) to the ref-store change.

This PR now contains only the universal "understanding" layer: KindOf/Kind, isULID (oklog-backed), Validate/NewCheckpointID/UnmarshalJSON, and the Pattern/CheckpointPattern matchers. No unused exports, no forward-referencing comments; the Kind type doc drops its git-ref clause.