stdio transport for barrel_mcp_client.

Spawns the configured executable as an Erlang port, frames stdin as line-delimited JSON-RPC, and forwards each complete line to the owning client gen_statem as {mcp_in, self(), Json}.

A line is "complete" when cowboy delivers {eol, _}; partial reads ({noeol, _}) are buffered until the matching {eol, _} arrives. The default 1 MiB line limit matches the previous in-process implementation.