Add geometry simplification to polygonize()

[brendancol]

Summary

• Add simplify_tolerance and simplify_method parameters to polygonize()
• Two algorithms: Douglas-Peucker (distance-based) and Visvalingam-Whyatt (area-based)
• Shared-edge decomposition means adjacent polygons get the same simplified boundary, so no gaps or overlaps
• All backends (numpy, cupy, dask+numpy, dask+cupy) and all return types

Closes #1151

Test plan

• DP kernel: straight line, bend preservation, sub-tolerance removal, two-point input
• VW kernel: straight line, large/small triangle area, two-point input
• Shared-edge topology: vertex reduction, area preservation, shared edge identity
• API validation: tolerance=None, tolerance=0, negative tolerance, bad method string
• Both algorithms reduce vertices on staircase boundaries
• Total area preserved after simplification
• column and polygon_points stay in sync when degenerate polygons get dropped
• All return types (geopandas, geojson, spatialpandas, awkward)
• Dask produces same areas as numpy for both DP and VW
• CuPy produces same areas as numpy
• Edge cases: holes, affine transform, single-pixel raster
• Backward compatible (all 77 existing tests still pass)