Add BigTIFF COG external compliance test and tier entry (#2303) (#2306)

Pins writer.bigtiff_cog at the ``advanced`` tier and lands the
external-interop gate that has to soak in CI before a future PR can
promote it to ``stable``. The compliance file exercises the BigTIFF
layout (8-byte offsets, magic 43, BigTIFF IFDs) by forcing
``bigtiff=True`` on a small raster, plus an auto-promotion row that
drives the UINT32_MAX threshold via the IFD-overhead helper rather
than allocating a multi-gigabyte buffer.

Closes #2303.

Author: brendancol

docs/source/reference/geotiff.rst

@@ -206,3 +206,25 @@ returned with holes left as the band's nodata sentinel (or zero on
 integer bands without a sentinel). ``XRSPATIAL_GEOTIFF_STRICT=1``
 forces the raise in ``'warn'`` mode too, so CI environments can enforce
 fail-fast behavior globally.
+
+BigTIFF COG (issue #2303)
+=========================
+
+A COG larger than the classic-TIFF 4 GiB offset ceiling needs the
+BigTIFF wrapper (magic ``43``, 8-byte offsets, 20-byte IFD entries).
+``to_geotiff(..., cog=True)`` auto-promotes to BigTIFF when the
+estimated file size exceeds ``UINT32_MAX`` (0xFFFFFFFF bytes); callers
+can force the wrapper with ``bigtiff=True`` even on small rasters when
+they want a stable layout for downstream tooling that probes the magic
+byte. The same threshold and force-flag rules apply whether the output
+is a plain GeoTIFF or a COG.
+
+``SUPPORTED_FEATURES['writer.bigtiff_cog']`` is currently ``advanced``.
+The external-interop gate lives in
+``xrspatial/geotiff/tests/test_bigtiff_cog_compliance_2286.py`` and
+covers the BigTIFF-specific layout (header, IFDs, tile and overview
+offset tables), one lossless integer codec, one lossless float codec,
+single-band and 3-band, one overview level, plus an auto-promotion row
+that drives the threshold via the IFD-overhead helper rather than
+allocating a multi-gigabyte buffer. Promotion to ``stable`` follows the
+same release-cycle soak rule as the rest of the COG surface.

xrspatial/geotiff/_attrs.py

@@ -290,6 +290,16 @@
     'writer.gpu': 'experimental',
     'writer.gdal_metadata_xml': 'experimental',
     'writer.extra_tags': 'experimental',
+    # BigTIFF COG writer surface (issue #2303, part of #2286 wave D).
+    # Tracked separately from ``writer.bigtiff`` and ``writer.cog`` because
+    # the BigTIFF + COG combination has its own external-interop surface
+    # (8-byte offsets in tile/overview tables, BigTIFF-form IFDs, COG
+    # layout invariants). Stays ``advanced`` even when every row of
+    # ``tests/test_bigtiff_cog_compliance_2286.py`` passes -- promotion
+    # to ``stable`` happens after the gate has lived in CI for a release
+    # cycle. See the BigTIFF COG section in
+    # ``docs/source/reference/geotiff.rst``.
+    'writer.bigtiff_cog': 'advanced',
 }