feat/v3 scale offset cast

[d-v-b]

This PR adds scale_offset and cast_value codecs. Used together, these codecs replace the fixedscaleoffset codec defined in numcodecs.

closes #3772

as an experiment, I'm including some developer notes I had claude write in the commit history. We can remove them before merging if we don't want that kind of thing sticking around.

[d-v-b]

cc @emmaualemalthot

[d-v-b]

this implementation is inefficient and will probably remain so as long as we are using python for this.

[d-v-b]

I'm a bit worried that this implementation is a bit too much stuff for zarr-python, and that perhaps this belongs in a separate project that manages the zarr-python compatibility. Thoughts?

[emmanuelmathot]

@d-v-b For me it really depends on how we can easily release this PR. How easy is it to release it here and then move to an external package or vice-versa?

[maxrjones]

I'm a bit worried that this implementation is a bit too much stuff for zarr-python, and that perhaps this belongs in a separate project that manages the zarr-python compatibility. Thoughts?

I don't see a lot of value in bundling it in zarr-python, since it seems unlikely that we'd long-term encourage a pure python implementation. I'd recommend a new repository in zarr-developers for easy findability.

[d-v-b]

@d-v-b For me it really depends on how we can easily release this PR. How easy is it to release it here and then move to an external package or vice-versa?

this PR will not be easy to merge, so I'm going to close it and move this functionality to a separate package at https://github.com/zarr-developers/cast-value.py/