Settled-design spec for the walk-forward param-plane cut: WalkForwardResult gains space: Vec<ParamSpec>, WindowRun.chosen_params Vec<Scalar> -> Vec<Cell>, walk_forward takes the space by value (Fn-closure generic untouched), and param_stability schema-checks a per-slot coercer (Bool -> 0/1, Timestamp unreachable per C20) before a type-blind reduction, deleting scalar_as_f64 + its per-value unreachable.
Behaviour-preserving (C1): param_stability yields identical MetricStats over the existing fixtures. Overturns 0047's Fork A (chosen_params stays Vec<Scalar>) — the WF structs carry no serde derives, so the serializability defense is moot; the substantive case is C7 purity + SweepFamily symmetry. grounding-check PASS.
refs #75
Brummel
791b809413