refactor: extract dask-specific functions to dask.py

Create new modules that re-export dask/cubed functions from core.py:
- flox/dask.py: dask_groupby_agg, dask_groupby_scan, etc.
- flox/cubed.py: cubed_groupby_agg

This improves code organization and provides a clearer API for
backend-specific functionality.

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

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

Author: dcherian

flox/cubed.py

@@ -0,0 +1,16 @@
+"""Cubed-specific functions for groupby operations.
+
+This module provides Cubed-specific implementations for groupby operations.
+Re-exported from flox.core for backward compatibility.
+"""
+
+from __future__ import annotations
+
+# Re-export from core for backward compatibility
+from .core import (
+    cubed_groupby_agg,
+)
+
+__all__ = [
+    "cubed_groupby_agg",
+]

flox/dask.py

@@ -0,0 +1,32 @@
+"""Dask-specific functions for groupby operations.
+
+This module provides Dask-specific implementations for groupby operations.
+Re-exported from flox.core for backward compatibility.
+"""
+
+from __future__ import annotations
+
+# Re-export from core for backward compatibility
+from .core import (
+    _collapse_blocks_along_axes,
+    _extract_unknown_groups,
+    _grouped_combine,
+    _normalize_indexes,
+    _simple_combine,
+    _unify_chunks,
+    dask_groupby_agg,
+    dask_groupby_scan,
+    subset_to_blocks,
+)
+
+__all__ = [
+    "_collapse_blocks_along_axes",
+    "_extract_unknown_groups",
+    "_grouped_combine",
+    "_normalize_indexes",
+    "_simple_combine",
+    "_unify_chunks",
+    "dask_groupby_agg",
+    "dask_groupby_scan",
+    "subset_to_blocks",
+]