Update py-hamt to 3.4.0

[Faolain]

Summary

Updates the client to use py-hamt 3.4.0 and adapts IPFS Zarr loading for the new sharded store behavior introduced in dClimate/py-hamt#86.

What changed

• Bumped py_hamt to >=3.4.0 and refreshed uv.lock to resolve py-hamt 3.4.0.
• Added optional zarr_group support to dClimateClient.load_dataset() and the legacy CID loader.
• Auto-opens group "0" when a py-hamt v2 sharded store reports multiple top-level groups and that safe default is available.
• Suppresses py-hamt's legacy v1 deprecation warning inside the client read path.
• Keeps post-sharded-open Zarr/group errors from being masked by the HAMT fallback.
• Documents the new zarr_group option and metadata field.

Why

py-hamt 3.4.0 adds path-aware sharded Zarr v2 support and can require explicit Zarr groups for pyramid/multi-group layouts. The client now handles the common group "0" layout automatically while still allowing callers to specify another group explicitly.

Validation

• uv run --extra testing pytest tests/test_ipfs_retrieval.py -q
• uv run --extra dev ruff check dclimate_client_py/ipfs_retrieval.py dclimate_client_py/dclimate_client.py dclimate_client_py/datasets.py tests/test_ipfs_retrieval.py

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/update-py-hamt-3-4-0

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