Update README dependency graph to match current setup.cfg (#1908)

* Update README dependency graph to match current setup.cfg (#1907)

Rewrite img/dependencies.dot and re-render the SVG and PNG so the
diagram reflects what xarray-spatial actually depends on today.

Required deps (green): numpy, xarray, numba, scipy, matplotlib,
zstandard.

Optional deps (yellow, dashed): cupy, cuspatial, rtxpy, kvikio,
nvcomp, dask, dask-geopandas, shapely, geopandas, awkward,
spatialpandas, pyproj, deflate.

Removed datashader, holoviews, rasterio, GDAL, GEOS, fiona, rtree,
libspatialindex, and pandas. The first four conflict with the
README's "Free of GDAL / GEOS Dependencies" claim and the built-in
GeoTIFF reader/writer; the rest are no longer in the tree.

* deps graph: address copilot review on legend colors + nvcomp naming

- Recolor required nodes ``aquamarine -> palegreen`` so the
  rendered diagram matches the PR description's "Required (green)"
  legend. The previous aquamarine fill drifted from the intended
  green and would mislead readers using the legend as a key.
- Rename the ``nvcomp`` node to ``libnvcomp``, update the label to
  ``libnvcomp.so``, and keep the official nvCOMP product page as
  the href. The optional dependency is the shared library
  ``libnvcomp.so`` loaded via kvikio/cuda; spelling it as
  ``nvcomp`` invited the misread that ``pip install nvcomp`` is the
  install instruction.
- Regenerate the SVG/PNG from the updated .dot so the rendered
  images match the source.

* docs: address PR #1908 review (note kvikio/libnvcomp install + cluster_ naming)

* docs: address Copilot review (rename nvcomp node, fix node fill)

Author: brendancol

README.md

@@ -677,6 +677,11 @@ Check out the user guide [here](/examples/user_guide/).
 - `kvikio` — GPUDirect Storage (SSD → GPU)
 - `fsspec` + `s3fs`/`gcsfs`/`adlfs` — cloud storage
 
+> `libnvcomp` and `kvikio` are not pulled in by the `gpu` extra. They are runtime
+> dependencies of the GeoTIFF GPU read path and must be installed separately
+> (typically via conda from the `rapidsai`/`nvidia` channels), since `libnvcomp`
+> ships as a system library and `kvikio` requires a matching CUDA toolkit.
+
 ![title](img/dependencies.svg)
 
 #### Notes on GDAL