Function auto_generated_chart_path 

pub fn auto_generated_chart_path(format: ChartFormat) -> PathBuf

Synthesize a unique path under std::env::temp_dir()/hyperdb-charts/ for a default-disposition chart write. The filename encodes a monotonic counter + PID + unix-nanos so two calls in the same nanosecond (or on two hosts with sync’d clocks) don’t collide.

The parent directory is not created here — the caller does that right before writing, to keep this function pure and cheap for testing.