test(cp0): anchor arc_grid invariant guard on __file__ (no vacuous pass)

tests/test_arc_grid_vendored.py

@@ -6,7 +6,10 @@ def test_arc_grid_engine_surface_importable():
 
 def test_arc_grid_has_no_squid_game_imports():
     import pathlib
-    root = pathlib.Path("proteus/arc_grid")
+    # Anchor on this test file's location, not the cwd, so the invariant can
+    # never pass vacuously (an empty rglob) when pytest runs from elsewhere.
+    root = pathlib.Path(__file__).parent.parent / "proteus" / "arc_grid"
+    assert root.is_dir(), f"vendored arc_grid not found at {root}"
     offenders = [
         p for p in root.rglob("*.py")
         if "squid_game" in p.read_text(encoding="utf-8")