COG readiness: external compliance test suite for to_geotiff(cog=True) (#2286 PR 2/6)

[brendancol]

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

Scope

Add an external-interop compliance suite that exercises COGs written by to_geotiff(..., cog=True) and asserts they are readable and well-formed under rasterio. No production code changes — tests only.

Required changes

New file: xrspatial/geotiff/tests/test_cog_writer_compliance.py.

Test matrix:

• Stable codecs only: none, deflate, lzw, zstd, packbits (skip if not currently interop-validated).
• Dtypes: at least one integer dtype (e.g. uint16) and one float dtype (e.g. float32).
• Band counts: single-band and multiband (3-band).
• Nodata: sentinel value and NaN cases.
• Georef: pixel-is-area and pixel-is-point.
• Overviews: explicit level list and auto-generated levels.
• Overview resampling modes that are intended to be stable.

For each row, assertions:

• rasterio.open succeeds.
• Base pixels equal expected (allclose with documented tolerance for lossy paths, byte-exact for lossless).
• Overview count and dims match.
• Overview pixels match expected within tolerance.
• CRS, transform, nodata, dtype, band count, and dim names survive the round-trip.
• TIFF layout is COG-like: tiled (not stripped), overview IFDs present, sane tile offsets, IFDs placed before image data.
• Optional rio-cogeo / GDAL validate_cloud_optimized_geotiff check that pytest.skips cleanly when the dependency is missing.

Out of scope

• Do not modify writer code. Failures should produce actionable test failures, not silently get patched over.
• GPU writer paths.
• Experimental codecs.
• BigTIFF beyond what existing tests already cover.

Acceptance

• New test file added under xrspatial/geotiff/tests/.
• Suite green on CPU CI.
• Any genuinely-broken row gets a targeted xfail with a linked issue rather than a silent skip.