refactor: centralize numeric normalization

tests/test_preservation_slice_screener.py

@@ -9,12 +9,15 @@ import advanced_strategies
 import llm_strategy
 import screener
 import strategy_tracking
+from backend.data.numbers import finite_number
 from backend.features.screener import compiler, engine, strategies, tracking
 from backend.features.screener import service as screener_service
 from tests.preservation_helpers import (
     assert_frontend_runtime_matches_audited_baseline,
     assert_moved_asset_matches,
     assert_page_prefix_matches,
+    function_contract,
+    module_contract,
 )
 
 
@@ -91,14 +94,6 @@ def top_level_definition(path: Path, name: str) -> str:
     return ast.dump(node, include_attributes=False)
 
 
-def module_without_imports(path: Path) -> str:
-    tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
-    tree.body = [
-        node for node in tree.body if not isinstance(node, (ast.Import, ast.ImportFrom))
-    ]
-    return ast.dump(tree, include_attributes=False)
-
-
 def sha256(path: Path) -> str:
     return hashlib.sha256(path.read_bytes()).hexdigest()
 
@@ -154,11 +149,22 @@ class ScreenerSliceSourceEquivalenceTests(unittest.TestCase):
 
     def test_engine_and_tracking_logic_match_the_original(self) -> None:
         self.assertEqual(
-            module_without_imports(ORIGINAL_ROOT / "screener.py"),
-            module_without_imports(
-                APP_ROOT / "backend" / "features" / "screener" / "engine.py"
+            module_contract(
+                ORIGINAL_ROOT / "screener.py",
+                excluded_definitions={"_number"},
+                exclude_imports=True,
+            ),
+            module_contract(
+                APP_ROOT / "backend" / "features" / "screener" / "engine.py",
+                excluded_definitions={"_number"},
+                exclude_imports=True,
             ),
         )
+        self.assertEqual(
+            function_contract(ORIGINAL_ROOT / "screener.py", "_number"),
+            function_contract(APP_ROOT / "backend/data/numbers.py", "finite_number"),
+        )
+        self.assertIs(engine._number, finite_number)
         self.assertEqual(
             class_methods(
                 ORIGINAL_ROOT / "backend" / "features" / "screener" / "tracking.py",