Consolidate backend-parity tests (epic #2390 PR 4)

[brendancol]

Background

The geotiff test suite has four backend-parity files plus several adjacent attrs-parity and kwarg-parity files that all cover roughly the same ground: does backend X (numpy / cupy / dask+numpy / dask+cupy / VRT / HTTP) match the eager-numpy reference on fixture Y. Combined line count is over 3,300.

PR 4 of the epic at #2390. Largest cluster by lines.

Scope

Fold into xrspatial/geotiff/tests/parity/:

• parity/test_backend_matrix.py for matrix-style parity across backends, dtypes, fixtures
• parity/test_pixel_equality.py for pixel-equality assertions (strictest mode)

Folded source files (deleted in the same commit):

• test_backend_parity_matrix.py
• test_backend_full_parity_2211.py
• test_backend_pixel_parity_matrix_1813.py
• test_backend_kwarg_parity_1561.py
• test_attrs_finalization_parity_2211.py
• test_attrs_parity_1548.py
• test_miniswhite_backend_parity_1797.py

Left for later PRs:

• test_vrt_backend_parity_2321.py, VRT-specific, picked up by PR 6

Constraints

Tests-only restructure. No source changes. Markers come from _helpers/markers.py; TIFF construction from _helpers/tiff_builders.py. Test ids are descriptive (id="float32-numpy-vs-cupy"), no issue numbers.

Audit

A temporary CLUSTER_AUDIT_PR4.md maps old to new test ids. Deleted in a final commit before approval.

Verification

pytest xrspatial/geotiff/tests/parity/ -v
pytest xrspatial/geotiff/tests/ -x -q