test: DFG Re20 steady rule per review (1 % band, range and slope, 0.5 s samples)

Samples every 0.5 s at synchronized root steps; trailing 5 s window (11
samples); range and |slope*window| each at most 1 % of the official band
width. States the fluid-on-cylinder force sign, reports cd_positive,
defaults to the refined case and uses the review's label.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit 71221837f9)

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, 2000))
-        self.assertEqual((uniform["steps"], uniform["interval"]), (480000, 4000))
+        self.assertEqual((root["steps"], root["interval"]), (240000, 4000))
+        self.assertEqual((uniform["steps"], uniform["interval"]), (480000, 8000))
         self.assertAlmostEqual(v.plan()["refined_fine_tau"], 0.62, places=12)
 
     def test_config_only_changes_viscosity(self):
@@ -47,17 +47,22 @@ class Re20Contract(unittest.TestCase):
     def test_steady_and_verdict(self):
         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"]), ("WITHIN_INTERVALS", True))
+        self.assertEqual((out["status"], out["steady"], out["window_samples"]),
+                         ("WITHIN_INTERVALS", True, 11))
+        self.assertTrue(out["cd_positive"])
         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 a tenth of its interval width inside the window.
-        out = v.summarize(rows(lambda i: dict(cd=5.58, cl=0.0106 + 1e-5 * i, p_diff=0.1174)))
+        # Cl drifts by more than 1 % of its interval width (6e-6) inside the window.
+        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
 
     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))