refactor: centralize numeric normalization

app/tests/test_preservation_slice_market_insights.py

@@ -6,11 +6,13 @@ import unittest
 from pathlib import Path
 
 import market_insights
+from backend.data.numbers import non_nan_number
 from backend.features.market import insights as canonical_insights
 from tests.preservation_helpers import (
     assert_frontend_runtime_matches_audited_baseline,
     assert_moved_asset_matches,
     assert_page_prefix_matches,
+    function_contract,
 )
 
 
@@ -106,6 +108,11 @@ class MarketInsightsSliceSourceEquivalenceTests(unittest.TestCase):
             MARKET_INSIGHT_METHODS,
         )
         self.assertIs(market_insights.MarketInsightsService, canonical_insights.MarketInsightsService)
+        self.assertEqual(
+            function_contract(ORIGINAL_ROOT / "market_insights.py", "_number"),
+            function_contract(APP_ROOT / "backend/data/numbers.py", "non_nan_number"),
+        )
+        self.assertIs(canonical_insights._number, non_nan_number)
 
     def test_dashboard_service_methods_are_exact_original_ast(self) -> None:
         original = ORIGINAL_ROOT / "server.py"