COG readiness: add COG rows to backend parity/release gate (#2286 PR 4/6)

[brendancol]

Part of #2286 (COG readiness/stability rollout).

Scope

Add a focused COG parity layer to the release gate so COG read/write paths are required-green for promotion.

Required changes

New file: xrspatial/geotiff/tests/test_cog_parity_2286.py (or an additive section to test_backend_full_parity_2211.py; prefer a new file to keep diffs reviewable).

Required rows:

1. xrspatial write COG -> xrspatial eager read
2. xrspatial write COG -> xrspatial dask read
3. xrspatial write COG -> rasterio read
4. golden/rasterio COG fixture -> xrspatial local read
5. golden/rasterio COG fixture -> xrspatial HTTP range read
6. golden/rasterio COG fixture -> xrspatial dask HTTP range read

For each row:

• Compare expected pixel values within documented tolerances (lossless = byte-exact).
• Assert CRS, transform, nodata, dtype, band count, dim names survive.
• Skip reasons must be explicit and pytest.skip with a string mentioning the dependency or linked issue. No silent skips.

Use the existing golden corpus under xrspatial/geotiff/tests/golden_corpus/ where it applies, or generate a small COG fixture via rasterio in the test if no suitable golden exists.

Out of scope

• GPU rows (reader.gpu stays experimental).
• Experimental codecs.
• Promoting any feature tier (PRs 5/6 do that).

Acceptance

• New file under xrspatial/geotiff/tests/.
• All six rows present, each either passing or skipping with a linked reason.
• Suite green on CPU CI.