✨ Implement xy_coords method on PyCudaCogReader struct

[weiji14]

Enable Python access the x- and y-coordinates derived from the affine transform.

Preview at https://cog3pio--82.org.readthedocs.build/en/82/api/#cog3pio.CudaCogReader.xy_coords

Cherry-picked from #71, continued from #70.

TODO:

• Implement xy_coords method on PyCudaCogReader struct
• Add unit tests

[codspeed-hq]

Merging this PR will degrade performance by 90.23%

❌ 1 (👁 1) regressed benchmark
✅ 6 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
👁	test_read_geotiff_remote	19.1 ms	195.7 ms	-90.23%

---

_{Comparing py/xy_coords (1e51abb) with main (0c688ff)}