refactor: extract dask-specific functions to dask.py and helpers to lib.py

Move dask-specific functions to flox/dask.py:
- dask_groupby_agg, dask_groupby_scan
- _grouped_combine, _simple_combine (via import)
- _unify_chunks, _normalize_indexes, subset_to_blocks
- _extract_unknown_groups, _collapse_blocks_along_axes

Move helper functions to flox/lib.py:
- identity
- _is_arg_reduction, _is_minmax_reduction, _is_first_last_reduction
- _is_bool_supported_reduction, _is_sparse_supported_reduction
- _is_reindex_sparse_supported_reduction

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: dcherian

flox/aggregate_sparse.py

@@ -8,7 +8,8 @@
 import numpy as np
 import sparse
 
-from flox.core import _factorize_multiple, _is_sparse_supported_reduction, factorize_
+from flox.core import _factorize_multiple, factorize_
+from flox.lib import _is_sparse_supported_reduction
 from flox.xrdtypes import INF, NINF, _get_fill_value
 from flox.xrutils import notnull