Avro and Protobuf wire codecs as optional plugins

[pathosDev]

The `Codec` interface (#6) ships with `jsonCodec` (pass-through) and `zodCodec` (validate via Zod schema). The original issue noted Avro / Protobuf as out-of-scope for v1.

Scope:

• `avroCodec(schema)` (peer-dep: `avsc` or similar)

  • `encode`: validate + serialize to Avro binary.
  • `decode`: deserialize + validate.
  • Wire shape: base64-encoded bytes inside the JSON envelope (so journal layers stay JSON-shaped).

• `protobufCodec(messageType)` (peer-dep: `protobufjs`)

  • Same shape: encode/decode via protobuf.js's `MessageType`.
  • Wire shape: base64.

• Both codecs play nicely with `SchemaRegistry` — register the codec instance with the version, the registry handles the rest.

Tests:

• Round-trip a value through the codec.
• Schema mismatch on decode (wrong magic, wrong fields) surfaces a clear error.
• Versioned via `SchemaRegistry`: v1 Avro codec + v2 Protobuf codec coexist (different versions of the same manifest can use different wire formats).

Estimate: 3-5 days.