Function parse_lua_blueprint 

pub fn parse_lua_blueprint(lua_src: &str) -> Result<Node, EngineError>

Load a Lua source, evaluate it, and pull out a BluePrint Node (mlua_flow_ir::Node). The Lua source must ultimately return a table — one that follows the flow.ir schema.

Example:

return {
  kind = "seq",
  children = {
    { kind = "step", ref = "agent-a",
      in_  = {op="path", at="$.input"},
      out  = {op="path", at="$.mid"} },
    ...
  }
}

Note: the serde tag is kind for Node and op for Expr; field names are the same snake_case as the Rust struct. ref is a reserved word, so the Lua key stays ref and serde renames ref ↔ ref_; the same holds for in.