Function write_slice_metadata_json 

pub fn write_slice_metadata_json(
    path: &Path,
    limits: (f64, f64),
    scale: ScaleMode,
    variable: &str,
    units: Option<&str>,
    long_name: Option<&str>,
    standard_name: Option<&str>,
    time_label: Option<&str>,
    depth_index: usize,
) -> Result<(), Box<dyn Error>>

Write machine-readable metadata alongside the raster export. This keeps variable names, CF/COARDS attributes, limits, and slice coordinates intact even in workflows that only ingest PNG files.