Move `Domain.dx` to `OnGrid.dx`

[astanziola]

The .dx attribute only make sense for fields defined on a grid, and can be uniquely found from the shape of OnGrid.params and the size of the domain.

It is much more natural to define the domain as

domain = Domain(size: tuple)

# Or, potentially
domain = Domain.from_grid(N, dx)

It would be even better to define domains that are not rectangular, for example

domain = Domain()  # Abstract domain

class RectangularDomain(Domain):
  L: tuple

class SphericalDomain(Domain):
  R: float

which then allows to define non standard discretizations. For example, Continuous can work on arbitrarily shaped domains, FourierSeries probably only makes sense on RectangularDomain while something like a SphericalFourierSeries (see for example s2fft) could be implemented on a SphericalDomain.

This is clearly a breaking change.

[astanziola]

Additional motivation: Domain.dx is currently static=True in the equinox module, which means JAX arrays cannot be stored there (JAX 0.9.x raises ValueError: arrays cannot be passed as metadata fields). This came up in jwave's CBS normalization where arithmetic on dx produced JAX arrays that were then passed back to Domain().

Longer-term: making dx a traced leaf would enable differentiating through grid spacing and avoid the static metadata limitation. All current usages of dx (FFT freq axes, kernel scaling, spatial axes) are JAX-compatible and would work with traced values.