geotiff tests: consolidate GPU writer cluster (Sub-PR B)

[brendancol]

Closes part of #2438 (Sub-PR B). Cluster 14 of long-tail epic #2424.

Summary

Fold 8 GPU writer test files into a new xrspatial/geotiff/tests/gpu/test_writer.py:

• test_gpu_writer_attrs_1563.py (write-path attrs parity)
• test_gpu_writer_band_first_1580.py ((band, y, x) layout remap + auto-dispatch)
• test_gpu_writer_compression_modes_2026_05_11.py (zstd, deflate, jpeg, none)
• test_gpu_writer_cpu_fallback_codecs_2026_05_12.py (lzw, packbits, lz4, lerc, jpeg2000)
• test_gpu_writer_nan_sentinel_1599.py (NaN-to-sentinel rewrite)
• test_gpu_writer_overview_inplace_1948.py (cupy.putmask in COG loop)
• test_gpu_writer_overview_mode_and_compression_level_1740.py
• test_to_geotiff_gpu_fallback_1674.py (gpu=True exception classification)

Tests-only. The shared requires_gpu marker from _helpers/markers.py replaces the per-file _gpu_available / _gpu_only helpers. 87 tests collected, matching the baseline sum.

The release-gate writer.gpu row and the experimental GPU codec note in docs/source/reference/geotiff.rst now cite gpu/test_writer.py.

Verification

• pytest xrspatial/geotiff/tests/gpu/test_writer.py -v: 87 passed on a CUDA-equipped host.
• pytest --collect-only xrspatial/geotiff/tests/gpu/test_writer.py: 87 tests collected.
• pytest xrspatial/geotiff/tests/release_gates/test_stable_features.py: 158 passed, 1 xfailed.

Test plan

• New consolidated gpu/test_writer.py collects and runs locally.
• Release-gate checklist-parity test passes.

Companion PRs

One of four sub-PRs against issue #2438. Sub-PRs A (gpu/test_reader.py), C (gpu/test_codec.py), and D (gpu/test_kernels_and_kwargs.py) are independent; merge order does not matter.

[brendancol]

PR Review: geotiff tests: consolidate GPU writer cluster (Sub-PR B)

Blockers

None.

Suggestions

None.

Nits

• xrspatial/geotiff/tests/gpu/test_writer.py:37 -- importlib.util is imported at module scope but the only remaining caller is _nvjpeg_available. The original per-file _gpu_available helpers also used it; here the GPU detection delegates to _helpers/markers.py, so it only feeds the nvjpeg probe. Could be moved inside _nvjpeg_available, but a stdlib import at module scope is the more conventional choice and I would leave it.
• xrspatial/geotiff/tests/gpu/test_writer.py:1398 -- the source-locate path in test_gpu_writer_overview_loop_uses_putmask_1948 now walks parent.parent.parent / "_writers" / "gpu.py" (the original used parent.parent). The audit file calls this out, but a one-line comment in the test docstring would put the explanation where the next reader actually lands. Optional.

What looks good

• Every test name from the eight source files is present in the new file. 68 unique functions, 87 parametrise-expanded cases, matching the pre-consolidation baseline.
• _make_int_da (Section 3, 64x64 int32) and _make_int_da_small (Section 4, 32x32 uint16) are deliberately kept distinct so the test inputs each section saw stay the same. Merging the two helpers would have changed inputs.
• Per-file _gpu_available / _gpu_only helpers are replaced with the shared requires_gpu marker from _helpers/markers.py, matching the convention from #2470. The fallback-section tests keep no @_gpu_only decorator on purpose (they monkeypatch write_geotiff_gpu), as they did originally.
• Release-gate citations in docs/source/reference/release_gate_geotiff.rst and docs/source/reference/geotiff.rst point at the new file. pytest xrspatial/geotiff/tests/release_gates/: 158 passed, 1 xfailed locally.
• CLUSTER_AUDIT_GPU_B.md maps every old test to its new location and will be deleted in the final pre-merge commit per the epic's hard gate.

Checklist

• All original test names migrated
• Tests-only PR as scoped (no source changes)
• requires_gpu marker correctly applied
• Path resolution in source-locate test verified (walks to _writers/gpu.py)
• Helper defaults preserved per source file
• Release-gate citation updated; release-gate parity test passes
• Audit file created for the epic gate

[brendancol]

Follow-up review

Picked up at commit 1e286ff.

Blockers

None.

Suggestions

None.

Nits

None remaining. The earlier nit on the source-locate path docstring was addressed in 1e286ff with a short path note. The importlib.util module-scope import was kept (conventional placement for a stdlib import; the helper that uses it is defined later in the file).

Re-run

• pytest xrspatial/geotiff/tests/gpu/test_writer.py: 87 passed.
• pytest xrspatial/geotiff/tests/release_gates/: 158 passed, 1 xfailed.

Ready as-is from the review side. Merging gated only on CI and the final pre-merge audit-file deletion per the epic #2424 hard gate.