Expose GeoTIFF loader as an xarray backend engine

[brendancol]

Summary

The native GeoTIFF/COG/VRT loader is currently available through xrspatial.geotiff.open_geotiff(...) and the .xrs accessors, but it does not appear to be registered as an xarray backend engine. It would be useful to open GeoTIFF sources through xarray's standard backend API:

import xarray as xr

xr.open_dataset("dem.tif", engine="xrspatial_geotiff")
xr.open_mfdataset("*.tif", engine="xrspatial_geotiff")

Current behavior

The package metadata only registers the xrspatial console script under [options.entry_points]; there is no xarray.backends entry point. I also could not find a BackendEntrypoint implementation in the tree.

The documented API is explicit import/use of open_geotiff:

from xrspatial.geotiff import open_geotiff, to_geotiff

open_geotiff("dem.tif")
open_geotiff("dem.tif", chunks=512)
open_geotiff("dem.tif", gpu=True)

open_geotiff returns a DataArray, while xarray backend engines are expected to open a Dataset.

Requested behavior

Add an xarray backend entry point, probably something like:

[options.entry_points]
xarray.backends =
    xrspatial_geotiff = xrspatial.geotiff._xarray_backend:GeoTIFFBackendEntrypoint

with a BackendEntrypoint wrapper around open_geotiff(...) that returns a one-variable Dataset.

Design notes / questions

• Engine name: xrspatial_geotiff, geotiff, or something else?
• Variable naming: use open_geotiff(..., default_name=...), name=, source stem, or xarray's default data variable naming conventions?
• Forwarding kwargs: map chunks, gpu, masked, dtype, band, overview_level, window, bbox, stable_only, etc. through backend_kwargs.
• Multi-file behavior: verify xr.open_mfdataset(..., engine=...) works naturally once each file opens as a compatible Dataset.
• Autodetection: optionally implement guess_can_open for .tif, .tiff, .vrt, COG URLs, and supported fsspec URIs.

Why

This would let users compose xrspatial's no-GDAL GeoTIFF support with standard xarray workflows and avoid having to import xrspatial explicitly just to access the loader.