Add raster geometric operations (polygon clipping, rotation, seam blending)

[brendancol]

Some basic raster manipulation operations are missing from xarray-spatial.

Scope

Polygon clipping

Clip a raster to an arbitrary polygon geometry, not just a bounding box like crop() does. Pixels outside the polygon get masked to nodata. This should accept shapely geometries or coordinate arrays for the clip mask.

This is probably the highest priority item here. It comes up constantly in analysis workflows where you need to extract data for an irregular study area, watershed boundary, administrative unit, etc.

Raster rotation and affine transformation

Arbitrary rotation and affine transforms beyond what reprojection handles. Useful for aligning imagery, rotating to a local coordinate frame, or correcting sensor geometry.

Mosaic with feathered blending

Distance-weighted blending at seam lines when mosaicking overlapping rasters. The existing merge() handles overlap by picking one value, but doesn't blend. Feathered blending produces much better results where rasters overlap, especially when there are brightness or exposure differences between tiles.

Notes

Polygon clipping involves rasterizing the polygon boundary and masking. The existing rasterize() function could potentially be reused for the polygon-to-mask step.