proximity: add return annotations to allocation and direction

[brendancol]

Closes #2717

allocation and direction now carry the same -> xr.DataArray return annotation that proximity already has. The three functions share a signature and return type, so their annotations should match.

• Annotation only: no behavior, signature, or default changes. Not a breaking change.
• Adds a parametrized test asserting the three siblings report the same return annotation via inspect.signature.

Backend coverage: unaffected (no runtime code changed). numpy / cupy / dask+numpy / dask+cupy paths are untouched.

Test plan:

• pytest xrspatial/tests/test_proximity.py::test_return_annotation_consistency (3 passed)
• proximity/allocation/direction default-behavior slice (17 passed)
• flake8 on changed files (no new findings)

[brendancol]

Domain-aware review (API-consistency sweep, proximity module).

Blockers: none.

Suggestions: none blocking. The annotation widens to match proximity's existing -> xr.DataArray, which is the right consistency target. Note that all three are wrapped by @supports_dataset and can return a Dataset, so a stricter xr.DataArray | xr.Dataset would be more precise. Matching the existing proximity annotation is the correct scope here; broadening all three is a separate call and out of scope for this PR.

Nits: none.

Verified:

• inspect.signature reports xr.DataArray for all three functions (the @supports_dataset wrapper preserves the signature via functools.wraps), so the new test exercises the real public surface.
• Annotation-only change; no runtime path touched. numpy / cupy / dask+numpy / dask+cupy behavior unchanged.
• New test and the proximity/allocation/direction default-behavior slice pass; flake8 reports no new findings on the changed files.