pad trace matrices to same height in the `main` sumcheck phase

[kunxian-xia]

The zerocheck expression becomes

$$0 = \sum_{b \in H_n} f(m_0(b), m_1(b), \ldots, m_w(b)) * \textrm{sel}(b)$$

where the selector polynomial $\textrm{sel}(x)$ is 1 at the real rows and 0 at the remaining padding rows.