Function decode_parallel 

pub fn decode_parallel(
    shared_secret: &[u8],
    packet: &KkParallelPacket,
) -> Result<Vec<u8>>

Decode a parallel-encoded packet, verifying the Merkle root and each chunk.

Steps:

1. Recompute the Merkle root from chunk commitments
2. Verify it matches the packet’s stored root (detects reorder/tamper)
3. Decrypt all chunks in parallel
4. Concatenate plaintext in order