List a serving deployment's generation chain (platform admin)

The revision history: this deployment's ancestry and its descendants in the same lane, each with the field diff against its parent, class labeled. Terminal generations are included, because they ARE the history: "what changed between the generation that worked and this one" is the question the chain exists to answer. env diffs report key names only. Returns the whole chain in one response: the walk is bounded at 50 generations, so there is nothing to page through.