Function close_traced 

fn close_traced(
    trace: &mut TurnTrace,
    known_over: bool,
    keys: &mut HashMap<String, usize>,
    turns: &mut [TurnSpan],
    open: Option<usize>,
)

Name the turn a transcript’s own rows support, then start the next turn’s trace. A turn the wire already settled is named by its own result frame and this one nothing, so the two never name one turn twice.

known_over is whether whatever closed the turn says so - the next prompt. A turn closing at the end of a conversation has nothing saying it, and needs its own last frame to have stopped rather than handed back for a tool call.