G28 -- Refuse unsupported input; a helper refuses an empty work list

A tool must refuse an input it cannot handle, loudly, naming the tool that can — never process it wrongly. A helper given an empty list refuses rather than building the unchanged tree and exiting zero. provenance: BFM R43 + R68; a gater that accepted a binary class it could not gate (105 cards, 0 banked); the no-op bank helper