Stale golden-corpus skips hide external-overview validation on dask/fsspec/GPU backends

[brendancol]

Describe the bug

The golden-corpus backend parity modules list overview_external_ovr_uint16 in _OVERVIEW_READER_GAPS, which skips the overview-level oracle comparison for the dask, fsspec, GPU, and dask+GPU backends:

• xrspatial/geotiff/tests/golden_corpus/test_dask_numpy.py:81
• xrspatial/geotiff/tests/golden_corpus/test_fsspec.py:87
• xrspatial/geotiff/tests/golden_corpus/test_gpu.py:87
• xrspatial/geotiff/tests/golden_corpus/test_dask_gpu.py:85

The gate switches off the candidate_factory that reads each overview level via open_geotiff(..., overview_level=lvl). The base-IFD parity check still runs, but the overview levels stored in the sibling .tif.ovr are never checked on these backends.

The eager numpy module cleared its own _OVERVIEW_READER_GAPS entry once the reader gained external .ovr sidecar discovery (_sidecar.py). That sidecar code is shared across backends, so the four remaining skips look stale and leave overview reads unverified on every backend except eager numpy.

Expected behavior

Backends whose read path resolves the sidecar pyramid should run the overview-level comparison for overview_external_ovr_uint16, the same as eager numpy. A skip should only remain where a backend genuinely cannot serve the level.

To reproduce

Drop the overview_external_ovr_uint16 entry from _OVERVIEW_READER_GAPS in a backend module and run that backend's golden-corpus parity test for the fixture id.