Optionally validate on deserialization

[kylecarow]

Depended on #36 (merged): splits Strategy::init into a pure validate and a
mutating init, and adds validate_strategy()/init_strategy() inherent methods to
each Interp*D. That split is what lets this issue call only the cheap check on
deserialize, without touching init at all, matching #36's own design intent of
decoupling the two rather than tethering them back together here.

Currently, deserializing an Interp*D struct populates fields directly without calling validate(). This means:

• LinearUniform: a deserialized interpolator with a non-uniform grid will silently produce wrong results at query time
• Step: a mismatched direction-count will pass validation silently

Proposed change

Use a proc macro to generate a *Wire helper struct for each interpolator. The real Deserialize impl uses #[serde(try_from = "Interp1DWire<D, S>")], and TryFrom calls data.validate() and validate_strategy(), both cheap, pure &self checks. On success, the interpolator is fully validated. On failure, deserialization returns an error instead of silently constructing broken state.

This does not call init_strategy(). That stays exactly what it already is today: a manual, opt-in method for anyone who needs guaranteed-fresh derived state, whether after deserializing a stateful custom strategy or after mutating data/strategy directly. Nothing about this issue changes that contract, on either the checked path or the deserialize_unvalidated escape hatch below. A strategy that serializes its own derived state (e.g. a future CubicSpline's precomputed coefficients) gets it back as-is, trusted, exactly the point of having serialized it in the first place; a strategy that skips a derived field via its own #[serde(skip, default)] already requires the caller to call init_strategy() manually, same as today.

This requires a proc-macro crate, which Rust requires to be separate from the crate using it ([lib] proc-macro = true can't coexist with regular library code). That means converting ninterp into a workspace with a new ninterp-macros crate, versioned in lockstep: the same split serde/serde_derive and thiserror/thiserror-impl use.

Escape hatch

Expose Interp1D::deserialize_unvalidated(raw: Interp1DRaw<D, S>) -> Self for callers who need to bypass validation (performance-critical hot paths, staged construction, etc.). The Interp1DRaw type is the plain derived struct: same wire format, no validation.

Not named deserialize_unchecked: that was considered and rejected on #18 for interpolate_fast, since _unchecked in Rust convention pairs with unsafe fn and implies UB on misuse (get_unchecked, from_utf8_unchecked). This is a logic-error contract, not a soundness one, the same reasoning as interpolate_fast over interpolate_unchecked. It's named for the thing it actually skips.

deserialize_unvalidated skips data.validate() and validate_strategy(). Since neither path ever touches init_strategy(), this is the only difference between the two: cheap invariant checks, on or off. This formalizes today's default behavior (deserialize populates fields, no validation) as an explicit, named opt-out, once validated deserialization becomes the default.

Behavior summary

	Current	Proposed
Invalid data from external source	Silent wrong results	Deserialization error
Valid round-trip (serialize -> deserialize)	Works	Works (validate passes trivially)
Stateful custom strategy needing fresh derived state	Caller must call init_strategy() manually	Unchanged: caller must still call init_strategy() manually
Bypass available	N/A	deserialize_unvalidated (skips data.validate()/validate_strategy() only)