Scalar was `struct { kind: ScalarKind, cell: Cell }` — "a Cell wearing a kind hat." The recorded reason for that shape was migration ease, which is not a design rationale (CLAUDE.md: rationale != effort), so the struct had no substantive defense. Redefine it as the native tagged union it conceptually is:
enum Scalar { I64(i64), F64(f64), Bool(bool), Timestamp(Timestamp) }
This is substantively better on four axes: kind/bits skew becomes unrepresentable (illegal states gone); accessors panic loudly on the wrong variant instead of a release-mode silent bit reinterpret; PartialEq derives the documented IEEE-754 / cross-kind value semantics (the hand-roll, needed only because the struct inherited Cell's bitwise compare, is gone); and serde is a plain derive emitting the externally-tagged wire form ({"I64":10}/{"F64":2.5}) — the private ScalarRepr shadow enum that motivated this was never needed. It is also C7-honest: erased-on-the-hot-path (Cell) and self-describing-at-the-edge (Scalar) are two disjoint types bridged by explicit conversion.
The whole public API is preserved (Scalar's fields were private), so call sites do not churn: the enum change is contained to scalar.rs, where cell() now encodes and from_cell() decodes per kind. Cell and ScalarKind are untouched.
With Scalar serializable, lift RunManifest.params from Vec<(String, f64)> to Vec<(String, Scalar)>: the param's kind (an i64 length vs an f64 scale) now survives into the C18 record (runs.jsonl) and the CLI JSON instead of collapsing to f64. scalar_as_param_f64 is deleted; sim_optimal_manifest passes typed params through. This is a deliberate wire-shape change — params now render as tagged scalars; the JSON-asserting tests are updated to the new shape on purpose.
Hand-authored manifest params across the CLI's single-run/mc/macd sites use honest kinds (lengths -> i64, scales -> f64) so they match the sweep path (which already derives correct kinds via zip_params); their in-binary JSON assertions are re-tagged accordingly.
Walk-forward fork resolves with no code change: WindowRun.chosen_params stays Vec<Scalar> (the serializable record carrier), reduced to f64 only at the param_stability statistic boundary (scalar_as_f64 retained). Glossary 'cell' entry updated to describe Scalar as the disjoint tagged union, not 'a cell plus its kind tag'.
Gates: build --all-targets, test --workspace (incl. new scalar_serde_round_trips), clippy -D warnings, doc --no-deps — all clean.
Brummel
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