Array API: decide a policy for inherently CPU-only operations

[mwcraig]

Background

Several ccdproc operations depend on libraries that are inherently CPU/numpy-only. For non-numpy input they either densify silently (Dask, JAX) or error (CuPy). Affected:

Function	CPU-only dependency
cosmicray_lacosmic	astroscrappy (C extension)
wcs_project	reproject
median_filter, background_deviation_filter, cosmicray_median internals, ccdmask (non-block branch)	scipy.ndimage
block_reduce, block_average, block_replicate	astropy forces np.asanyarray on inputs

Decision needed

Pick one policy and apply it consistently:

1. Convert + warn: explicitly convert to host memory, run the operation, convert the result (data and mask/uncertainty) back to the input namespace, and emit a warning the first time.
2. Raise: refuse non-numpy input with an informative error telling the user to convert explicitly.

Silent densification is the one unacceptable option (CuPy turns it into a confusing error deep inside the dependency anyway). Whichever policy wins should be documented in docs/array_api.rst and reflected in per-backend test markers.

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Follow-up to #909; relevant to #910 / #912. Found during a review of the array API implementation from #885.