test: DFG Re20 steady rule as agreed (10 % range, 2 % slope, window-mean verdict)

Supersedes 71221837. Samples every t* 0.5 (interval-mean forces), trailing
t* 50..60 window (21 samples); range <= 10 % and |slope*window| <= 2 % of
each official band width. Verdict on the window mean with the distance to
the nearest band edge. Our engineering rule, frozen before any run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit 869d60d78e)

tests/unit/test_dfg_re20.py

@@ -21,8 +21,8 @@ class Re20Contract(unittest.TestCase):
         root, uniform = v.lattice_parameters(40), v.lattice_parameters(80)
         self.assertAlmostEqual(root["tau"], 0.56, places=12)
         self.assertAlmostEqual(uniform["tau"], 0.62, places=12)
-        self.assertEqual((root["steps"], root["interval"]), (240000, 4000))
-        self.assertEqual((uniform["steps"], uniform["interval"]), (480000, 8000))
+        self.assertEqual((root["steps"], root["interval"]), (240000, 2000))
+        self.assertEqual((uniform["steps"], uniform["interval"]), (480000, 4000))
         self.assertAlmostEqual(v.plan()["refined_fine_tau"], 0.62, places=12)
 
     def test_config_only_changes_viscosity(self):
@@ -48,21 +48,27 @@ class Re20Contract(unittest.TestCase):
         inside = dict(cd=5.58, cl=0.0106, p_diff=0.1174)
         out = v.summarize(rows(lambda i: inside))
         self.assertEqual((out["status"], out["steady"], out["window_samples"]),
-                         ("WITHIN_INTERVALS", True, 11))
+                         ("WITHIN_INTERVALS", True, 21))
         self.assertTrue(out["cd_positive"])
+        self.assertAlmostEqual(out["edge_distance"]["cl"], 0.0002)
         self.assertTrue(all(out["verdict"].values()))
         low = v.summarize(rows(lambda i: dict(inside, cl=0.0100)))
         self.assertEqual(low["status"], "OUTSIDE_INTERVALS")
         self.assertFalse(low["verdict"]["cl"])
 
-    def test_drift_gives_no_verdict(self):
-        # Cl drifts by more than 1 % of its interval width (6e-6) inside the window.
+    def test_slow_drift_gives_no_verdict(self):
+        # Range 2e-5 passes (limit 6e-5); slope * window 2e-5 exceeds 1.2e-5.
         out = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106 + 1e-6 * i, p_diff=0.1174)))
         self.assertEqual((out["status"], out["verdict"]), ("NOT_STEADY", None))
-        self.assertGreater(out["ranges"]["cl"], out["steady_limits"]["cl"])
-        self.assertGreater(out["drifts"]["cl"], out["steady_limits"]["cl"])
-        small = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106 + 2e-6 * (i % 2), p_diff=0.1174)))
-        self.assertTrue(small["steady"])  # 2e-6 alternation, no trend: within both limits
+        self.assertLessEqual(out["ranges"]["cl"], out["range_limits"]["cl"])
+        self.assertGreater(out["drifts"]["cl"], out["slope_limits"]["cl"])
+
+    def test_noise_range(self):
+        quiet = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106 + 2e-5 * (i % 2), p_diff=0.1174)))
+        self.assertTrue(quiet["steady"])  # within the range limit, no trend
+        noisy = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106 + 1e-4 * (i % 2), p_diff=0.1174)))
+        self.assertEqual(noisy["status"], "NOT_STEADY")
+        self.assertGreater(noisy["ranges"]["cl"], noisy["range_limits"]["cl"])
 
     def test_incomplete_run_gives_no_verdict(self):
         out = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106, p_diff=0.1174), tstar_end=30))