Add geotiff writer test matrix (#1483)

[brendancol]

Summary

Test-only coverage for geotiff write findings T-5, T-6, T-7, T-9 from a recent audit. No source changes. New file xrspatial/geotiff/tests/test_writer_matrix.py adds 85 tests.

• T-5: dtype x compression matrix. 8 dtypes (uint8/16/32, int16/32/64, float32/64) x 5 codecs (none, deflate, lzw, zstd, lz4) round-tripped through stripped and tiled layouts on 32x32 rasters.
• T-6: NaN-vs-sentinel nodata semantics. Float NaN nodata, float sentinel (-9999), uint8 sentinel (255). Verifies on-disk sentinel and read-back NaN behaviour through both read_to_array and open_geotiff.
• T-7: COG validity. Uses rasterio (skipped if missing) to confirm tiled layout, presence and monotonicity of overviews, and pixel round-trip. If rio-cogeo is installed, also runs cog_validate. Note: xrspatial does not currently emit GDAL's LAYOUT=COG IMAGE_STRUCTURE tag, so that specific marker is not asserted.
• T-9: write-to-readonly directory. Chmod 0o555 a temp dir, expect OSError/PermissionError, restore mode in finally. Skipped on Windows and when running as root.
• T-10: planar config 2 round-trip was dropped because the writer does not emit PlanarConfiguration=2.

Issue: #1483

Test plan

• pytest xrspatial/geotiff/tests/test_writer_matrix.py (85 passed locally with rasterio installed)
• Existing test_writer.py still passes
• CI run (rasterio-dependent COG test will skip if rasterio not in CI env)