fix(eigen): reject strides that are not a multiple of the scalar size

[henryiii]

🤖 AI text below 🤖

EigenProps::conformable divided the numpy byte strides by sizeof(Scalar) with integer
division. A stride that is not a whole number of scalars was truncated. A field of a packed
structured array (for example np.zeros(4, dtype=[('a', 'f8'), ('b', 'i1')])['a'], which has a
stride of 9 bytes) was thus seen as contiguous, and a mutable Eigen::Ref read and wrote the
neighbouring field.

The conformability result now records such strides. A mutable Eigen::Ref refuses the array, and
by-value and const Ref arguments copy it, as they already do for negative strides.

Fixes item 5 of #6159.

Suggested changelog entry:

• Fixed Eigen matrix casters accepting numpy arrays whose stride is not a multiple of the scalar
  size, which let a mutable Eigen::Ref corrupt neighbouring data.