fix: handle N base in bit-packed insertion encoding (#109)

When nst_nt4_table returns 4 (for ‘N’ or unknown bases), the bit shift can cause unexpected behavior:

1. In short insertions (line 318): value 4 encoded into 2-bit field
2. In long insertions (line 350): value 4 shifted by 0-6 bits can overflow uint8_t when bit_index=3 (4 << 6 = 256)

Fix: Check if base lookup returns >= 4 and substitute a random base, consistent with the behavior when bases parameter is NULL.