atproto_codegen/emit/union

Union synthesis with $type-dispatching codecs. Open unions get an Other(Dynamic) escape hatch; closed unions fail decode instead.

The three parts (type, encoder, decoder) are returned separately rather than as one joined string: the types plugin renders the first two, the decode-json plugin the third, and the host interleaves them with the def’s own type/encoder/decoder so the emitted file matches the order the monolithic emitter used to produce.

pub type UnionParts {
  UnionParts(type_def: String, encoder: String, decoder: String)
}

pub fn emit_union(
  plan: zero.ZeroPlan,
  cfg: config.Config,
  nsid: String,
  name: String,
  refs: List(String),
  closed: Bool,
) -> UnionParts