Brummel
9febdb9623
Make the walk-forward param plane C7-clean and symmetric with the sweep param plane. WalkForwardResult gains `space: Vec<ParamSpec>` (the kinds, once, mirroring SweepFamily.space) and WindowRun.chosen_params changes Vec<Scalar> -> Vec<Cell> (the tag-free coordinate the inner sweep already produces). param_stability now resolves its numeric coercion once per slot against the schema (result.space) into a Vec<fn(Cell)->f64>, then runs a type-blind reduction loop — replacing the old per-window-value scalar_as_f64 with its per-value unreachable!. scalar_as_f64 is deleted.
This overturns cycle 0047's "Fork A" (chosen_params stays Vec<Scalar>), which was justified by Scalar's new serializability. That defense was moot: WalkForwardResult and WindowRun carry no serde derives — they are never serialized; only the embedded oos_report (a RunReport, typed via the 0047 lift) is. So chosen_params is a pure in-memory substrate for param_stability, and Vec<Scalar> made it C7-impure (carrying the kind N×M times at the value, where the kind is per-slot constant). The substantive case — kind at the schema, SweepFamily symmetry, per-value unreachable! eliminated — wins, the same argument-form that retired the {kind,cell} Scalar struct in 0047.
The seam ([A]): walk_forward / walk_forward_with_threads take `space: Vec<ParamSpec>` by value, a sibling of the run-window closure; the Fn(WindowBounds)->WindowRun+Sync bound is unchanged and run_indexed is untouched (space is never captured by the Sync closure, moved onto the result at the end). A debug_assert in walk_forward_with_threads guards the arity coupling (every window's chosen_params.len() == space.len(), guaranteed by the same blueprint). walkforward_family computes the space once before the roll and passes the tag-free winner (chosen_params: best.params) directly, dropping the from_cell+zip reconstruction.
[D]: a Bool param slot coerces to 0/1 (mean = fraction "on"), keeping the coercer total over the knob kinds; a Timestamp slot is structurally impossible (C20) so the schema pass carries one unreachable! (once per slot, never per value). param_stability keeps an `if result.windows.is_empty() { return Vec::new(); }` guard to preserve its documented empty-on-no-windows contract. WalkForwardResult and SweepFamily share the (space + tag-free Cell points + zip_params) idiom but stay distinct types — different axis (Sweep enumerates input coordinates; WF rolls time and chosen_params is the optimizer's output), and WF additionally carries bounds + oos_equity + stitched_oos_equity.
Behaviour-preserving (C1): the three migrated walk-forward tests stay green with identical MetricStats (param_stability_reduces_chosen_params_per_slot still pins mean 2.5 / p50 2.5 / mean 1.5); the i64->f64 and f64 coercions are value-identical to the deleted scalar_as_f64. The one genuinely new behaviour (Bool 0/1) is unexercised by built-in grids and pinned by the new param_stability_reduces_bool_slot_to_fraction_true (mean 0.75, 3-of-4 true).
Gates: build --workspace --all-targets, test --workspace, clippy --workspace --all-targets -D warnings, doc --workspace --no-deps — all clean.
closes #75