Fix test collection errors: name collision and import path

- Add __init__.py files to all test directories so pytest uses
  package-based module naming (avoids flat name collisions)
- Rename tests/unit/calibration/test_puf_impute.py to
  test_calibration_puf_impute.py to avoid collision with
  tests/unit/test_puf_impute.py
- Fix test_version_manifest.py import: policyengine_us_data.tests.conftest
  no longer exists, use tests.conftest instead

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: anth-volk

tests/__init__.py

@@ -20,7 +20,7 @@
     get_data_manifest,
     get_data_version,
 )
-from policyengine_us_data.tests.conftest import (
+from tests.conftest import (
     make_mock_blob,
     setup_bucket_with_registry,
 )