Make shapely an optional [vector] extra (#2496)

[brendancol]

Closes #2496

shapely (and GEOS) loaded on every import xrspatial because rasterize.py
imported it at module top level, even though only rasterize and polygonize
use it. This moves it to an optional vector extra, matching the matplotlib
change in #2494.

• Replace the top-level import shapely in rasterize.py with a cached
  _require_shapely() helper. Each function that uses the shapely array API
  binds the module locally, so dask tile workers (which call the tile helpers
  directly, not through rasterize) also get the clear error. rasterize()
  calls it up front so a missing install fails early with a message pointing at
  pip install xarray-spatial[vector].
• Remove shapely>=2.0 from install_requires; add a vector extra and keep
  shapely in the tests extra so CI still exercises the vector paths.
• polygonize already imports shapely lazily, and only on its geopandas
  return path (which needs geopandas anyway), so it needs no change.
• Update README, install docs, and CHANGELOG.

This is a packaging change, not backend-specific. The rasterize backends
(numpy / cupy / dask+numpy / dask+cupy) are unchanged; shapely is only used for
geometry classification and coordinate extraction, which feed all backends.

Test plan

• New test_optional_shapely.py (3 passed): import xrspatial works in a
  subprocess with shapely blocked; _require_shapely() and rasterize() raise
  the friendly error pointing at the vector extra.
• pytest xrspatial/tests/test_rasterize.py xrspatial/tests/test_polygonize.py
  (356 passed, 15 skipped) with shapely installed.
• Confirmed import xrspatial no longer loads shapely (shapely absent
  from sys.modules) and a real rasterize still produces correct output.

[brendancol]

PR Review: Make shapely an optional [vector] extra (#2496)

Packaging change plus lazy-import plumbing. The rasterize algorithm and its
four backends are untouched, so the numeric/backend checklist mostly doesn't
apply here. I focused on whether every shapely use path is actually guarded.

Blockers

None.

Suggestions

None blocking.

Nits

• polygonize.py still does a bare import shapely inside _to_geopandas
  (around line 609). Without shapely that raises a plain ImportError rather than
  the [vector]-pointing message. In practice geopandas is imported first on
  that same path and geopandas requires shapely, so the user hits geopandas's
  own error first -- but the messaging is slightly inconsistent with the new
  rasterize behavior. Optional to route it through _require_shapely for a
  uniform message.
• test_optional_shapely.py::test_import_xrspatial_without_shapely imports
  xrspatial.rasterize in the subprocess but not xrspatial.polygonize. Adding
  polygonize to that import list would lock in that it also imports clean
  without shapely.

What looks good

• Every bare shapely. reference (13 of them) sits in a function that binds
  shapely = _require_shapely() first; rasterize() calls the guard up front
  for an early, clear failure. Verified each call site against its enclosing
  function.
• The dask path is handled correctly: tile workers call _polys_from_wkb /
  _classify_geometries directly, not through rasterize(), and those helpers
  bind via _require_shapely() so the worker either imports shapely or raises
  the friendly error. The cache lives in a module global, so each worker
  process resolves it once.
• Confirmed import xrspatial no longer loads shapely (absent from
  sys.modules), and a real rasterize still produces correct output.
• shapely kept in the tests extra, so CI still exercises the vector paths;
  356 rasterize/polygonize tests pass.
• New tests cover the subprocess import, the helper message, and the up-front
  rasterize() error, and they reset the _shapely cache so the in-process
  checks are real.

Checklist

• Every shapely use path guarded (rasterize + dask helpers)
• import xrspatial no longer loads shapely
• Friendly ImportError points at the vector extra
• tests extra keeps shapely so CI exercises vector paths
• Docs (README, installation.rst) and CHANGELOG updated
• polygonize geopandas path uses the same [vector] message (nit)
• No backend/algorithm change; existing rasterize tests pass

[brendancol]

Follow-up review (after 7047f03)

Disposition of the two nits from the first pass:

• Nit 2 (subprocess import test): fixed. test_import_xrspatial_without_shapely
  now also imports xrspatial.polygonize with shapely blocked, locking in that
  it imports clean too. 3 passed.
• Nit 1 (route polygonize's geopandas path through _require_shapely):
  dismissed. On that path import geopandas runs first, and geopandas
  hard-requires shapely, so a "geopandas installed but shapely missing" state is
  not reachable. Adding a cross-module import for a message that can't surface
  isn't worth it.

No remaining items beyond CI.