use scverse-misc for dataset registry and downloads

[PPSherry]

Implements #472 by moving dataset metadata into snapatac2/datasets.yaml and routing downloads through scverse-misc[datasets].

• Add YAML registry for all 22 dataset files.
• Use scverse-misc for registry parsing and fetching.
• Preserve existing snap.datasets.* and snap.genome.* APIs.
• Add runtime fallback to original URLs if the new URL fails.
• Bump Python requirement to >=3.12.
• Drop Python 3.11 wheel builds.

[PPSherry]

tests/test_pipeline.py::test_backed currently fails with a backed-mode complex128 panic in spectral(). This appears unrelated to the dataset migration and will be filed separately.