Bite-sized, placeholder-free plan for the walk-forward axis, in three tasks:
(1) extract MetricStats::from_values from McAggregate::from_draws + serde derive
(behaviour-preserving, the 7 mc tests guard); (2) the walkforward module —
WindowRoller (bounds-only iterator), walk_forward over the shared run_indexed
core, continuous OOS stitch (empty segment -> +0.0), on-demand param_stability,
+ crate exports + module-header doc-lag fix; (3) the aura walkforward CLI demo
bridging engine+registry via optimize, with serde_json for the summary line.
RED-first tests 1-11 (incl. 7b empty-segment stitch) per the spec.
refs #69
Brummel
7d52eff41f