RON serde does not round-trip fixed-`N` interpolator data

[kylecarow]

InterpDataNDBase round-trips through RON. Every fixed-N type fails, in both the
plain and Nested serialization formats:

Data1D   derived    FAIL 2:11: Expected opening `[`
Data1D   nested     FAIL 2:11: Expected opening `[`
DataND   derived    OK
DataND   nested     FAIL 2:11: Expected opening `[`
Interp1D derived    FAIL 3:15: Expected opening `[`
Interp1D nested     FAIL 3:15: Expected opening `[`

Cause

InterpDataBase<D, N> stores its grid as [ArrayBase<D, Ix1>; N], a fixed-size array.
Serde treats that as a tuple, so RON writes (...), but deserialize_fixed asks for a
sequence and RON's deserializer then requires [...]. InterpDataNDBase is unaffected
because its grid is a Vec<ArrayBase<D, Ix1>>, which serializes as a sequence either way.

The Nested path fails for the same reason: GridArrWrapper also wraps the fixed-size
array.

Repro

Add ron as a dev-dependency, then:

let data = Interp1D::new(
    array![0., 1., 2.],
    array![0.0, 0.4, 0.8],
    strategy::Linear,
    Extrapolate::Error,
)
.unwrap()
.data;

let s = ron::to_string(&data).unwrap();
ron::from_str::<ninterp::data::InterpData1D<f64>>(&s).unwrap(); // panics

Scope

Pre-existing, not a regression: reproduces identically on main and on every released
version that has the fixed-N grid. JSON and bincode are unaffected, since neither
distinguishes tuples from sequences on the wire. Not currently covered by
tests/serde_formats.rs, which tests JSON and bincode only.

Possible fixes

• Serialize the fixed-size grid as a sequence rather than a tuple (a custom
  serialize_with on the grid field, mirroring what deserialize_fixed already
  expects). Changes the RON output shape; leaves JSON and bincode output identical.
• Accept both tuple and sequence input in deserialize_fixed, leaving the output shape
  alone. Fixes reading but leaves RON output asymmetric with InterpDataNDBase.
• Document RON as unsupported and leave it.

Worth deciding alongside whether tests/serde_formats.rs should cover a third,
self-describing format at all.