Persist material efficiency snapshots

tests/test_material_efficiency.py

@@ -152,14 +152,15 @@ def test_temporal_regression_and_successive_feedback():
     assert first.good_quantity_m2 == 800
 
 
-def test_naive_feedback_requires_explicit_timezone_and_preserves_source():
+def test_naive_feedback_requires_explicit_timezone_and_returns_instant():
     naive = datetime(2026, 9, 4, 12, 5)
     status = replace(STATUS, feedback_timestamp=naive)
     with pytest.raises(ValueError, match='explicit erp_timezone'):
         service().evaluate(status)
     subject = service(erp_timezone=ZoneInfo('Europe/Berlin'))
     result = subject.evaluate(status)
-    assert result.erp_feedback_timestamp == naive
+    assert result.erp_feedback_timestamp == FEEDBACK
+    assert status.feedback_timestamp == naive
     assert subject.materials.latest_at_or_before.call_args.kwargs['timestamp'] == FEEDBACK
 
 
@@ -171,10 +172,11 @@ def test_dst_gap_and_overlap_rejected(timestamp):
         )
 
 
-def test_aware_feedback_preserves_offset():
+def test_aware_feedback_returns_utc():
     timestamp = FEEDBACK.astimezone(ZoneInfo('Europe/Berlin'))
     result = service().evaluate(replace(STATUS, feedback_timestamp=timestamp))
-    assert result.erp_feedback_timestamp is timestamp
+    assert result.erp_feedback_timestamp == timestamp
+    assert result.erp_feedback_timestamp.tzinfo is UTC
 
 
 def test_database_failure_propagates():