Protobuf codec for the Hrana 3 shared structures.

Filo's execution core works on the canonical Hrana JSON-term maps (the same maps Jason produces/consumes — integers carried as strings, blobs as no-pad base64, values tagged by "type"). Protobuf is therefore a pure edge codec: these functions convert between protobuf wire bytes and those same maps, so the streams/dispatch/execution layers never change. The HTTP and WebSocket message envelopes are layered on top in Filo.Protobuf.Http and Filo.Protobuf.Ws.

Each encode_*/1 returns the message body as iodata/0 (a container embeds it with Filo.Protobuf.Wire.field_len/2); each decode_*/1 takes the message bytes and returns the Hrana map. Field numbers follow the proto3 schema in the Hrana 3 spec.