Update README dependency graph to match current setup.cfg

[brendancol]

Reason

The dependency graph at img/dependencies.dot (rendered to dependencies.svg and dependencies.png, shown in the README) is out of date. It still shows datashader, holoviews, rasterio, GDAL, GEOS, fiona, rtree, libspatialindex, and a shapely-via-fiona chain. None of those are runtime dependencies anymore:

• xrspatial/ does not import datashader, holoviews, or rioxarray in the main package. datashader only appears in the examples extra.
• The README advertises "Free of GDAL / GEOS Dependencies" and ships its own GeoTIFF/COG reader and writer.
• The required dep set per setup.cfg is numba, scipy, xarray, numpy, matplotlib, zstandard.
• New optional runtime deps are missing from the chart: kvikio (GPUDirect Storage tile reads) and nvCOMP (GPU decompression), both lazily loaded in xrspatial/geotiff/_gpu_decode.py.

Proposal

Rewrite img/dependencies.dot and re-render dependencies.svg and dependencies.png so the README image reflects what the project actually depends on.

Required (green, solid edges) — six entries from install_requires:

• numpy, xarray, numba, scipy, matplotlib, zstandard

Optional (yellow, dashed edges) — from [options.extras_require] and lazy imports:

• GPU compute: cupy, cuspatial, rtxpy
• GPU I/O acceleration: kvikio, nvcomp
• Distributed: dask, dask-geopandas
• Vector geometry: shapely, geopandas, awkward, spatialpandas
• Reprojection: pyproj
• GeoTIFF compression: deflate

Removed from the chart:

• datashader, holoviews (only in examples extra)
• rasterio, rioxarray, GDAL (replaced by built-in GeoTIFF reader/writer)
• GEOS, fiona, rtree, libspatialindex (no longer in the stack)
• pandas (transitive only; not used directly by xrspatial)

Flat layout — no Graphviz clusters, dashed edges from xarray_spatial to each optional, matching the style of the current diagram.

Value

The chart in the README is one of the first things a new contributor or user reads. Showing nodes that are not actually in the dep tree is misleading and contradicts the "Free of GDAL / GEOS" claim a few lines below it.

Files to Touch

• img/dependencies.dot
• img/dependencies.svg
• img/dependencies.png