Cycle 3 of the cost-model-graph milestone: lift the duplicated cost-node
skeleton (shared verbatim by ConstantCost/VolSlippageCost, locked by convention
against CostSum) into a CostNode factor trait + a generic CostRunner<F> adapter,
with one source of truth for the 3-field cost record. Behaviour-preserving: same
schemas, same wiring, byte-identical output; the existing suite is the regression
net. Cost-graph composite-builder deferred (decision E).
Grounding-check PASS (7/7 assumption groups against named green tests).
Fork decisions logged on the reference issue.
refs #148
