Function save_metrics 

pub fn save_metrics(
    metrics: &SessionMetrics,
    session_dir: &Path,
) -> Result<(), Error>

Incrementally write session metrics to session_metrics.json.

Merges with any existing file first: a second writer over the same session (TUI session switch, a new process resuming it) must extend the per-turn history rather than clobber it — a plain overwrite once erased a whole session’s stats when a resume started a fresh accumulator over the same directory. Union is keyed on (turn_number, started_at), so the per-turn saves from a single process stay idempotent.