Emits an Elixir module for a FlatBuffers union declaration.

The module owns the discriminator ↔ variant mapping and the two per-direction dispatch helpers:

• decode_variant/3 — given a discriminator integer and the referenced position in the buffer, return {variant_atom, value}.
• build_variant/3 — given a variant atom and a value, write it into the buffer and return {builder, addr} so the calling table can add (discriminator, uoffset) as a pair of fields.

Variant 0 is always the implicit :NONE.