geotiff: extract GPU helpers into _backends/_gpu_helpers.py (#1813 step 6) #1884

Reason

Six GPU-only helpers live in xrspatial/geotiff/__init__.py (_is_gpu_data L861, _apply_nodata_mask_gpu L878, _gpu_decode_single_band_tiles L2483, _apply_orientation_gpu L2569, _apply_orientation_geo_info L2616, _gpu_apply_window_band L2679). They are leaves of the GPU backend, and lifting them out lets the next PR move read_geotiff_gpu (about 615 lines, the file's largest single function) cleanly without re-extracting helpers mid-flight.

Proposal

Create the _backends/ subpackage (empty __init__.py) and add xrspatial/geotiff/_backends/_gpu_helpers.py with the six helpers above. Lazy-import cupy at call time (mirroring current behaviour) so the module imports on numpy-only environments. Re-import from __init__.py for the still-inline read_geotiff_gpu body.

Refs: #1813.