Experiment: a JSON twin with a semantic graph for each new piece

For new pieces only (the next batch), the desk also writes <piece>.json: the markdown, the sources and images it depends on, and a graph of typed nodes (Evidence, Idea, Question, Statement, Fact, Hypothesis, Observation, Comment) with edges (supports, contradicts, asks, answers, cites), the graph the prose was written from (fractal semantic graphs). The build renders the graph beside the piece and validates the schema. Two rounds, then decide.