9a29f079ef
Group Protocol fields by direction: `body: { schema, from }` for the
request side and `stream: { event, initial, step, terminal, onHalt }`
for the response side. Rename `Protocol.define`→`Protocol.make` and
`Protocol.jsonChunk`→`Protocol.jsonEvent` to match the rest of the
codebase. Type aliases follow: `*Payload`→`*Body`, `*Chunk`→`*Event`.