Respect masked array pixels in zonal statistics

[shkyyy18]

Fixes #246.

zonal_stats previously converted the input window to a plain ndarray and constructed a new geometry/nodata mask, discarding the source np.ma.MaskedArray mask. Masked pixels then contributed to count, sum, mean, and nodata/NaN counts. Preserve the source mask through boundless reads and combine it with the geometry and nodata masks.

Validation:

• Three synthetic zonal regression tests failed before the fix and pass after it (including masked nodata/NaN and an entirely masked zone).
• Four boundless source-mask cases failed before the fix; all eight parameterized cases pass after it in isolated synthetic checks.
• Five targeted zonal tests pass after the fix, including the existing NaN/nodata cases; Ruff check/format and git diff --check pass.
• The full local test suite and direct tests/test_io.py collection could not run because the installed pyogrio DLL is blocked by local Windows application policy. The five synthetic boundless_array checks ran independently of the vector fixture. CI remains unverified until it runs upstream.

Implemented and tested with Codex assistance; reviewed the patch and results before submission.