The Protocol Buffers (PBC) side of the fake Riak endpoint.

Every message is framed as <<length::32-big, msg_code::8, payload>> where length counts the code byte plus the protobuf payload. Supported messages:

Any other code gets an RpbErrorResp (0) and the connection stays open. Objects are stored as their raw RpbContent.value in the same bucketed FakeRiak.SimpleStore space the HTTP API uses, so the two APIs see each other's writes. Vector clocks are a static opaque token (clients only ever echo them back) and siblings are never returned.

This module is not wired to a listener itself: FakeRiak.Riak detects PBC framing on the first byte of a connection and delegates here, passing its own dispatch state through untouched (requests are independent, so no state of our own is needed).