refactor: enforce canonical backend imports

tests/test_feature_boundaries.py

@@ -20,12 +20,6 @@ class FeatureBoundaryTests(unittest.TestCase):
         }
         violations = []
         for path in FEATURES.rglob("*.py"):
-            # The screener engine is an exact-preservation move of the legacy
-            # calculation module.  Its provider dependency is covered by the
-            # slice equivalence tests and will be addressed only after the
-            # behavior-preserving migration is complete.
-            if path.relative_to(FEATURES).as_posix() == "screener/engine.py":
-                continue
             tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
             for node in ast.walk(tree):
                 names = []
@@ -38,6 +32,53 @@ class FeatureBoundaryTests(unittest.TestCase):
                         violations.append(f"{path.relative_to(ROOT)} -> {name}")
         self.assertEqual(violations, [])
 
+    def test_backend_uses_root_compatibility_modules_only_at_declared_boundaries(self) -> None:
+        compatibility_modules = {
+            "advanced_strategies",
+            "alert_service",
+            "api_access",
+            "app_config",
+            "assistant_agent",
+            "chart_data_provider",
+            "heaven_agent",
+            "heaven_engine",
+            "ifind_client",
+            "llm_strategy",
+            "llm_stream",
+            "market_insights",
+            "mentor_agent",
+            "realtime_aggregator",
+            "screener",
+            "security",
+            "sentiment_engine",
+            "server",
+            "strategy_tracking",
+            "trade_journal",
+            "tushare_client",
+        }
+        allowed = {
+            "backend/application.py": {"api_access"},
+            "backend/features/screener/repository.py": {"sentiment_engine"},
+        }
+        violations = []
+        for path in (ROOT / "backend").rglob("*.py"):
+            relative = path.relative_to(ROOT).as_posix()
+            tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
+            for node in ast.walk(tree):
+                names = []
+                if isinstance(node, ast.Import):
+                    names = [alias.name for alias in node.names]
+                elif isinstance(node, ast.ImportFrom) and node.module:
+                    names = [node.module]
+                for name in names:
+                    root_name = name.split(".")[0]
+                    if (
+                        root_name in compatibility_modules
+                        and root_name not in allowed.get(relative, set())
+                    ):
+                        violations.append(f"{relative} -> {name}")
+        self.assertEqual(violations, [])
+
     def test_legacy_service_modules_are_compatibility_exports_only(self) -> None:
         for filename in ("alert_service.py", "trade_journal.py", "strategy_tracking.py"):
             tree = ast.parse((ROOT / filename).read_text(encoding="utf-8"))