Pure SSE event framing.

Formats pre-encoded JSON-RPC payloads as Server-Sent Events per the MCP Streamable HTTP transport specification. No process state, no side effects — just string formatting. JSON encoding happens at the push entries (Wymcp.Session.push/3, Wymcp.Session.await_client_response/4) in the pusher's own process; by the time a payload reaches this module it is a JSON binary, which cannot contain a raw newline (JSON escapes control characters) — so the SSE data: line cannot be broken by content, by construction rather than by an unenforced precondition.

SSE Format

Each event has an optional ID (for resumability) and a data field containing a JSON-RPC message:

id: <event-id>
data: <json>

Events are separated by a blank line (\n\n).