areev-grain-type

Checklist for adding a new grain type or changing an existing one's fields in areev-core — registry row, serialize/deserialize downcast arms, compact field-map keys, omit-default rules, and the conformance/roundtrip tests. Use whenever you touch anything under crates/areev-core/src/types/ or add a grain type byte, because serialization dispatches on downcast chains and a registry row alone silently half-works.