wellen split-bit merge drops late-starting bits and X values

Beyond the start-offset bug in wellen 0.25.7's transform_bv_signal (merging bit-blasted VCD vars like data [0], data [1] into data[1:0]), two more defects: an input whose first change is later than the other bits' gets a None offset and is never advanced (maybe_next_time and the advance loop only consider Some offsets), so that bit's values are lost entirely; and on_change derives its state count only from inputs that have values, so the X for a not-yet-valued bit is written as 0 in a 2-state merge. Fix: treat a None offset's next time as its first change, advance None to position 0, and join States::Four whenever any input lacks a value. A Rust test that reads a tiny VCD via wellen::simple::read and compares iter_changes catches all three.