Function heatmap_payload_bytes 

pub fn heatmap_payload_bytes(
    cells: u64,
    label: &str,
    detector: &str,
    advisory: Option<&str>,
) -> Option<u64>

Calculate the checked scalar-and-metadata payload footprint recorded in a scene/heatmap node.

The footprint intentionally describes the caller-prepared scalar payload, not a codec-specific serialized size. Surface projections use it as the stable byte budget across Scene codecs.