Message-level app/proto conversion spec.

Adapters bridge PB's native message shape and a caller-chosen application term such as a tagged tuple, struct, or scalar. Each adapter registers two {module, function} MFAs:

• :to_proto — converts an application value to a message field map.
• :from_proto — converts a message field map back to the application value.

Callbacks must return {:ok, value} | {:error, reason}.

Adapter conversion is one message level only. PB owns recursion into child messages, so child message values in both callback directions use the schema's native representation. In particular, :to_proto must emit native child values, not already-converted protobuf maps for those children.

adapter =
  %PB.Adapter{
    to_proto: {MyApp.TimestampCodec, :to_proto},
    from_proto: {MyApp.TimestampCodec, :from_proto},
    type: quote(do: DateTime.t())
  }

PB.compile(descriptor_set,
  projections: [{:"google.protobuf.Timestamp", adapter: adapter}]
)

The optional :type field carries an Elixir typespec used for PB.Adapter-aware docs; it is otherwise inert. The fully-qualified message name an adapter targets is supplied by the surrounding :projections entry, not by the adapter struct itself.