Function decode_stable_cell_payload 

pub fn decode_stable_cell_payload<M: Memory>(
    memory: &M,
) -> Result<Vec<u8>, StableCellPayloadError>

Decode the raw value payload from an ic-stable-structures::Cell memory.

This helper is intentionally narrow: it recognizes the physical stable-cell envelope and returns the value bytes. It does not deserialize those bytes or decide whether they represent a valid allocation ledger.