Consolidate release-gate tests into single registry (epic #2390 PR 10)

[brendancol]

PR 10 of the GeoTIFF test consolidation epic (#2390).

Scope

Collapse every @pytest.mark.release_gate test (and every test currently
living in a test_release_gate_*.py file under xrspatial/geotiff/tests/)
into one registry file:

xrspatial/geotiff/tests/release_gates/
├── __init__.py
└── test_stable_features.py

The release process needs one audit point for "what does CI run when
validating release readiness". Today, that audit lives in 13 files. After
this PR, it lives in one.

Files folded

• test_release_gate_2321.py
• test_release_gate_attrs_contract.py
• test_release_gate_codec_round_trip_2341.py
• test_release_gate_codecs.py
• test_release_gate_cog.py
• test_release_gate_dask_parity.py
• test_release_gate_eager_dask_parity_2341.py
• test_release_gate_local_read.py
• test_release_gate_local_write.py
• test_release_gate_negative_2341.py
• test_release_gate_overview_sidecar_metadata_2341.py
• test_release_gate_windowed_read.py
• test_release_gate_windowed_reads_2341.py

All folded files are deleted in the same commit. Every test in the new
file keeps @pytest.mark.release_gate, grouped by feature area (read /
write / cog / codecs / parity / overview / windowed / negative).

Doc update

docs/source/reference/release_gate_geotiff.rst (touched by PR 1 of the
#2345 doc epic) cites the release-gate test files. The doc is updated to
point at the consolidated module, preserving the per-feature grouping in
prose even though the underlying file is one.

Constraints

• Tests-only restructure. No source changes.
• Markers from _helpers/markers.py; make_minimal_tiff from
  _helpers/tiff_builders.py.
• Pure-runtime test naming -- no issue numbers in test ids.
• Audit table CLUSTER_AUDIT_PR10.md lives alongside the new file during
  review, deleted before merge per epic protocol.

Verification

pytest xrspatial/geotiff/tests/release_gates/ -v -m release_gate
pytest xrspatial/geotiff/tests/ -x -q
pytest xrspatial/geotiff/tests/ -m release_gate -v

All must pass. -m release_gate should select the consolidated file and
no others.

Epic: #2390. Foundation PR (#2394) has merged; _helpers/ exists on
main.