Add preview() for memory-safe raster downsampling#987
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Uses xarray coarsen with block averaging for numpy and dask backends, stride-based subsampling for CuPy. Dask arrays stay lazy so peak memory is bounded by the largest chunk plus the output. Accepts DataArray or Dataset via @supports_dataset decorator.
Covers numpy, dask, cupy, dask+cupy, Dataset, NaN handling, block averaging correctness, passthrough for small rasters, input validation, and accessor integration.
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Summary
Closes #986.
preview()function that downsamples a DataArray (or Dataset) to a target pixel size using block averaging viaxarray.coarsen. For dask arrays the operation stays lazy, so a 30 TB raster can be previewed at 1000x1000 without blowing memory..xrsaccessor for both DataArray and Dataset.__init__.pyexports and API reference docs.Test plan