Add hotspots() and emerging_hotspots() dask+cupy backends

[brendancol]

Problem

Both hotspots() and emerging_hotspots() support 3 of 4 backends but are missing dask+cupy:

# focal.py line 1223
dask_cupy_func=lambda *args: not_implemented_func(
    *args, messages='hotspots() does not support dask with cupy backed DataArray.'),

# emerging_hotspots.py line 601
dask_cupy_func=lambda *args: not_implemented_func(
    messages='emerging_hotspots() does not support dask+cupy backed DataArray.',

Why This Should Be Straightforward

• The cupy backends for both already exist (_hotspots_cupy, _emerging_hotspots_cupy).
• The dask+numpy backends also exist and use map_overlap.
• The same file (focal.py) already has a working dask+cupy implementation for mean() at line 80 that demonstrates the pattern.
• The missing piece is combining these: map_overlap with cupy chunk functions and cupy meta arrays.

Proposed Fix

• hotspots: implement _hotspots_dask_cupy() following the _mean_dask_cupy() pattern.
• emerging_hotspots: implement _emerging_hotspots_dask_cupy() similarly.
• Update README feature matrix: add ✅ for Hotspots dask GPU column.

Impact

Hotspot analysis is critical for spatial epidemiology, crime analysis, and environmental monitoring — compute-intensive operations where GPU provides the most benefit.

[brendancol]

#914