feat: back next questions with explicit graph unknowns

docs/v0.6-comparability-experiment.md

@@ -42,3 +42,7 @@ The repeated pattern appeared in four scenarios, so a small pre-contradiction co
 A comparison question is useful only if its answer advances the reasoning stage rather than merely adding more text.
 
 In the revenue-versus-cash scenario, the first question now confirms whether the figures are comparable, and the answer resolves that existing uncertainty instead of creating a parallel note. After that update, the engine progresses from comparability assessment to cautious relationship assessment and can select one broad non-expert follow-up question.
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+Every justified next question should correspond to an explicit unresolved graph node.
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+The earlier fallback-only path has now been removed from the normal successful progression. After comparability is resolved and a further investigation question is justified, the engine creates or reuses an explicit unresolved reasoning unknown and lets deterministic selection and question formulation proceed through the standard graph pipeline. A fallback is now only acceptable as an explicit failure case, not as the normal source of the next question.