Add sky-view factor function (#962)

[brendancol]

Summary

• Add sky_view_factor() — fraction of visible sky hemisphere per DEM cell
• All four backends: NumPy (@ngjit), CuPy (@cuda.jit), Dask+NumPy (map_overlap), Dask+CuPy
• Casts rays at n_directions azimuths out to max_radius cells, tracks max horizon angle per direction
• SVF = 1 - mean(sin(max_horizon_angle)) per Zakek et al. (2011)

What changed

• New module xrspatial/sky_view_factor.py — CPU kernel, CUDA kernel, Dask wrappers
• Export in xrspatial/__init__.py
• 25 tests: correctness (flat, valley, ridge, ramp), NaN handling, validation, cross-backend, Dataset support
• API docs in docs/source/reference/surface.rst
• User guide notebook examples/user_guide/22_Sky_View_Factor.ipynb
• README feature matrix row under Surface

Test plan

• pytest xrspatial/tests/test_sky_view_factor.py — 25/25 pass
• All four backends produce matching results
• Flat surface gives SVF=1.0, valley floor < 1, ridge summit > 0.9
• NaN propagation and single-cell raster work correctly
• Bad parameters raise errors

Closes #962