Fix NumPy 2 split_img compatibility

[david-hoffman]

Problem\nExisting tests fail under NumPy 2.x because dphtools.utils.split_img calls np.product, which was removed.\n\n## Desired behavior\nsplit_img should preserve existing behavior and pass the current split_img tests under NumPy 2.x.\n\n## Acceptance criteria\n- Existing split_img tests pass.\n- No tests are weakened or skipped.\n- Product behavior is unchanged except NumPy 2 compatibility.\n\n## Risk\nrisk:low\n