`runWithGraphAllocT` shouldn’t union with existing graph

[Roly Perera]

• show method for edge lists
• runWithGraphT and runWithGraphAllocT inconsistent – only the latter unions with existing graph..
• testGraph (and perhaps testTrace): default selection should be topOf to better exercise graph & round-trip ❌ interacts poorly with fwd_expect having no annotations
• input to bwd on graph should restrict to vertices g for same reason as fwd