Consolidate writer / COG / BigTIFF tests (epic #2390 PR 7)

[brendancol]

Consolidate the writer / COG / BigTIFF test cluster per epic #2390.

Scope

Create xrspatial/geotiff/tests/write/ with four files:

• write/test_basic.py — generic writer paths (compression, tiling, kwarg order, return path, layout monkeypatch).
• write/test_cog.py — COG writer compliance and overview/nodata combinations.
• write/test_bigtiff.py — BigTIFF threshold and COG compliance for big files.
• write/test_overview.py — overview level and nodata-aware overview tests.

Folds 26 source files. Tests-only restructure; no source changes.

Constraints

• Markers come from _helpers/markers.py.
• make_minimal_tiff from _helpers/tiff_builders.py.
• No issue numbers in filenames or parametrize IDs.
• HTTP-side COG tests, reader-side files, and VRT non-writer files are out of scope (other PRs in the epic).

Parent: epic #2390.