Built-in HTTP/1.1 + HTTP/2 server for the MCP HTTP transports.

A small acceptor pool on a single port, built on the h1 and h2 protocol libraries (no cowboy). Cleartext binds speak HTTP/1.1; TLS binds advertise ALPN [h2, http/1.1] and dispatch each connection to the negotiated protocol — so one URL serves both, like the cowboy listener it replaces.

Per accepted connection a process owns the socket, performs the TLS handshake (TLS only), starts an h1_connection/h2_connection in server mode and runs the owner loop. Each request is handled in its own process that reads the body, builds a barrel_mcp_http_engineResponder over h1/h2:send_response/send_data, and runs the engine.

The engine, not this module, holds the MCP protocol logic; this module is pure transport plumbing.