Macro entity 

macro_rules! entity {
    (
        $(#[$meta:meta])*
        pub struct $entity:ident(update $update:ident, id $id:ty) {
            registers { $( $(#[$rmeta:meta])* $rvar:ident => $rfield:ident : $rty:ty ),* $(,)? }
            namespaces { $( $nvar:ident => $nfield:ident : $nty:ty ),* $(,)? }
            constants { $( $(#[$cmeta:meta])* $cfield:ident : $cty:ty ),* $(,)? }
        }
    ) => { ... };
}

One entity kind from one field list: the struct and its Update enum, plus the Entity impl. Field classes: registers (writable leaves), namespaces (nested vocabularies addressed by descent), constants (creation-time values — no update variant). Serde variant names on the update enum are wire tags: written explicitly, never derived from field names (FlipLR is not FlipLr), never renamed or repurposed — deprecate and add instead. The struct’s field names are the document format’s keys.