rasterize: annotate geometries, columns, merge params (#2250)

[brendancol]

Closes #2250.

Summary

xrspatial.rasterize.rasterize annotated 13 of 16 public parameters plus the return type. The remaining three -- geometries, columns, merge -- were left bare even though the docstring already declared the intended types. IDE hints, mypy/pyright, and sphinx autodoc see a partial picture when annotations stop mid-signature.

This PR adds annotations matching the docstring:

• geometries: Any -- the docstring's "GeoDataFrame or iterable of (geometry, value)" spans the geopandas / dask_geopandas / list-of-pairs cases; both geopandas backends are optional deps, so a precise Union would require runtime imports of optional packages in the type expression. Any keeps the public contract honest without pulling those imports up to module level.
• columns: Optional[Sequence[str]] -- matches "list of str, optional".
• merge: Union[str, Callable] -- matches "str or callable, default 'last'". The precise Callable[[float, np.ndarray, int], float] signature stays in the docstring body under merge_fn.

No behaviour change. Annotations are additive.

Sweep context

Found by /sweep-api-consistency on 2026-05-21 against the rasterize module. Cat 3 (type hints / docstring), MEDIUM.

Cross-module drifts that the sweep also noted but did NOT file per template rules (cross-cutting drift gets filed against the module pair, not against rasterize alone):

• clip_polygon(nodata=...) vs rasterize(fill=...) -- same concept, different name.
• clip_polygon(name: Optional[str] = None) vs rasterize(name: str = 'rasterize') -- default-resolution convention drift.
• polygonize(column_name=...) vs rasterize(column=...) -- column-selector naming drift.

Test plan

• pytest xrspatial/tests/test_rasterize_signature_annot_2250.py -- new regression suite pins every public-signature param plus the return annotation. The 2250-specific test names the three offending params directly so a regression message points at the right gap.
• pytest xrspatial/tests/test_rasterize.py -- 206 passed, 2 skipped (unchanged from main).
• Smoke-tested rasterize(use_cuda=True, merge=callable, ...) on a CUDA-enabled host.

[brendancol]

Review: rasterize annotate geometries, columns, merge (#2250)

Blockers

None.

Suggestions

None.

Nits

• merge: Union[str, Callable] leaves Callable unparameterized. The docstring already spells out merge_fn(pixel, props, is_first) -> float64, so Callable[[float, np.ndarray, int], float] would match. PR body justifies the looser form; not worth churning.

Notes

• Diff matches the title: three annotations plus one import line.
• from __future__ import annotations is on at line 12, so annotations stay lazy strings. No runtime cost, no need to pull optional deps to module top. inspect.signature(rasterize) resolves all 16 params plus the return.
• geometries: Any is the right call. The true union spans geopandas / dask_geopandas / iterable-of-pairs, two of which are optional deps. A precise Union forces module-top imports or forward-ref strings, both worse for IDE hints.
• columns: Optional[Sequence[str]] is wider than the docstring's "list of str". That matches actual behaviour: the runtime only iterates, so tuple input already worked.
• Regression test covers the general invariant and names the three #2250 params explicitly, so a future failure points at the right gap.
• Sweep state CSV records the cross-module drifts (clip_polygon fill/nodata, name default, polygonize column_name vs column) for later sweeps, per template.

Nothing blocking.