Blocking newline-delimited JSON-RPC 2.0 loop over stdin/stdout (MCP stdio transport, protocol version 2024-11-05). One request per line in, zero or one response line out, flushed immediately. EOF on stdin ends the loop cleanly. Refuses to run in interactive sessions so an accidental call cannot hang the console. Meant to be launched via `Rscript tools/mcp_server.R`.