Two solutions to a homogeneous second-order recurrence are $\phi_n^{(1)} = 2^n$ and $\phi_n^{(2)} = 3 \cdot 5^n$. Calculate their Casoratian (discrete Wronskian) $W_n = \phi_n^{(1)} \phi_{n+1}^{(2)} - \phi_n^{(2)} \phi_{n+1}^{(1)}$ and determine linear independence.