Re-export open_geotiff and to_geotiff from the top-level xrspatial namespace

[brendancol]

Closes #3005

open_geotiff and to_geotiff were the only public functions you had to import from the subpackage. Now they re-export from the top level like everything else:

• Add both to xrspatial/__init__.py next to the other top-level re-exports.
• from xrspatial.geotiff import open_geotiff, to_geotiff still works (same objects).
• Note the alias in the GeoTIFF reference docs.

No functional change to the readers/writers, so backend coverage (numpy / cupy / dask+numpy / dask+cupy) is untouched.

Test plan

• test_top_level_reexports_are_the_subpackage_functions pins both spellings to the same objects.
• Existing test_api_consolidation.py suite still passes (14 passed).

[brendancol]

PR Review: Re-export open_geotiff and to_geotiff from the top-level xrspatial namespace

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

None.

Nits (optional improvements)

• The two new imports sit between the zonal and reproject blocks in xrspatial/__init__.py:144-145. The file isn't strictly module-sorted anyway (hydro is scattered too), so this is fine as is. A geotiff block grouped with the other I/O imports would read a touch better, but not enough to warrant a re-push.

What looks good

• Both spellings resolve to the same objects; xrspatial.open_geotiff is xrspatial.geotiff.open_geotiff holds.
• There's no top-level __all__ to keep in sync, so the re-export matches how every other public function is exposed.
• The contract test pins identity, not just importability, so it catches a future accidental shadow.
• No circular import: the geotiff subpackage doesn't reach back into the top-level package.
• Backward compatible. The subpackage import keeps working.
• Docs mention the alias in both the Reading and Writing sections.

Checklist

• Algorithm matches reference/paper: n/a (no algorithm change)
• All implemented backends produce consistent results: n/a (no backend change)
• NaN handling is correct: n/a
• Edge cases covered by tests: the identity test covers the contract
• Dask chunk boundaries handled correctly: n/a
• No premature materialization or unnecessary copies: n/a
• Benchmark exists or not needed: not needed
• README feature matrix updated: not applicable (no new function)
• Docstrings present and accurate: docs updated