Benchmarks: broaden the dataset pool beyond ARCO-ERA5 #234

Most of the geospatial suite profiles against ARCO-ERA5 (plus one Sentinel-2 scene and WeatherBench 2 in specific cases). Datasets with different characteristics (chunking layout, dtype mix, dimensionality, compression, coordinate regularity) would help catch regressions and engine-specific behavior that ARCO-ERA5 alone does not surface.

Context: #227 (comment)