disconnect unused clk from dff to prevent spurious sanity check fails

src/kyupy/sim.py

@@ -204,6 +204,7 @@ class SimOps:
                 assert len(n.ins) <= 4, f'Too many input pins for node {n}. Map circuit to KYUPY techlib first using resolve_tlib_cells().'
                 in_idxs = [n.ins[x].index if len(n.ins) > x and n.ins[x] is not None else self.zero_idx for x in [0,1,2,3]]
                 if n in ppio2idx:
+                    in_idxs[1] = self.zero_idx
                     if 'dff' in n.kind.lower() or in_idxs[0] == self.zero_idx:
                         in_idxs[0] = self.ppi_offset + ppio2idx[n]
                     else:  # this is a PO that is also used within the circuit. Treat as normal fork with ref counting. TODO: check strip_forks=True case for this.