geotiff: extract coord/transform helpers to _coords.py

[brendancol]

Part of #1813.

First slice of the xrspatial/geotiff/__init__.py split tracked in #1813. The full split lays out per-backend modules; this PR only does the lowest-risk extraction so it can land independently.

Summary

• Adds xrspatial/geotiff/_coords.py holding the shared GeoTransform-to-coord builders: coords_from_pixel_geometry, transform_tuple_from_pixel_geometry, coords_from_geo_info, geo_to_coords, transform_tuple, transform_from_attr, coords_to_transform.
• __init__.py re-imports those helpers under the same leading-underscore names (_geo_to_coords, _transform_tuple, _transform_from_attr, _coords_to_transform, _BAND_DIM_NAMES) so existing test imports keep working.
• Collapses six inline copies of the windowed coord / transform-tuple arithmetic in:
  • _populate_attrs_from_geo_info (windowed transform tuple)
  • open_geotiff (windowed eager coords)
  • read_geotiff_dask (windowed dask coords)
  • _gpu_apply_window_band (windowed GPU coords for tiled reads)
  • read_geotiff_gpu (windowed GPU coords for stripped reads)
  • read_vrt eager + _read_vrt_dask_internal (VRT coords and transform tuple)
• __init__.py shrinks by ~280 lines.

Public API is unchanged. Behaviour is unchanged: the helpers reproduce the existing branch-by-branch logic verbatim, including the has_georef=False integer-pixel fallback (#1710, #1753) and the windowed-origin clamp used by the VRT path.

Test plan

• pytest xrspatial/geotiff/tests/ — same 2507 passing / 11 pre-existing failing as baseline before the refactor (the 11 failures reproduce on main and are unrelated to this PR).
• pytest xrspatial/geotiff/tests/test_coords_1813.py -v — 13 new unit tests covering basic affine, windowed read, AREA vs POINT half-pixel shift, negative y-resolution north-up, no-georef fallback, and coords_from_geo_info wrapper variants.

[Copilot]

Pull request overview

Lowest-risk first slice of the xrspatial/geotiff/__init__.py split tracked in issue #1813. Extracts the GeoTransform-to-coord and transform-tuple helpers — previously inlined in six different backend read paths — into a new internal _coords.py module, and replaces each inline copy with a call to the shared helpers. Public API is unchanged, and pre-existing private names (_geo_to_coords, _transform_tuple, _transform_from_attr, _coords_to_transform, _BAND_DIM_NAMES) are re-exported under the same names so existing tests keep working.

Changes:

• New xrspatial/geotiff/_coords.py with coords_from_pixel_geometry, transform_tuple_from_pixel_geometry, coords_from_geo_info, and the moved geo_to_coords / transform_tuple / transform_from_attr / coords_to_transform helpers.
• __init__.py collapses six inline windowed-coord / transform-tuple snippets to calls into the new helpers (~280-line net reduction); also unifies the has_georef=False integer-pixel fallback used by #1710 / #1753 across all backends.
• New test_coords_1813.py with 13 unit tests covering area vs point, north-up vs south-up, windowed reads, no-georef fallback, and the coords_from_geo_info wrapper.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
xrspatial/geotiff/_coords.py	New module holding the shared coord / transform helpers.
xrspatial/geotiff/__init__.py	Replaces six inline coord/transform blocks with calls into _coords; re-exports helpers under their old underscore names.
xrspatial/geotiff/tests/test_coords_1813.py	New unit tests for the extracted helpers.

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