//! Engine-agnostic Streamable HTTP transport primitives.
//!
//! This module owns the protocol-level logic of the MCP Streamable HTTP
//! transport — JSON-RPC framing, SSE replay/dispatch, and the request/response
//! types that flow through engine adapters. Engines (Volga, Axum, custom)
//! implement [`engine::HttpEngine`] and call the free helpers in [`handlers`].
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