Clarify improving explanations for flat trajectories

src/eigenscript/runtime/explain.py

@@ -204,9 +204,12 @@ def explain_improving(
             )
         elif previous_radius is not None and current_radius is not None:
             delta = current_radius - previous_radius
-            direction = (
-                "decreasing (improving)" if delta < 0 else "increasing (not improving)"
-            )
+            if delta < 0:
+                direction = "decreasing (improving)"
+            elif delta > 0:
+                direction = "increasing (not improving)"
+            else:
+                direction = "unchanged (not improving)"
             print(
                 f"  └─ radius: {previous_radius:.4f} → {current_radius:.4f}",
                 file=sys.stderr,

tests/test_explain.py

@@ -140,6 +140,21 @@ def test_explain_improving(self):
         assert "radius: 10.0000 → 8.0000" in output
         assert "decreasing (improving)" in output
 
+    def test_explain_improving_flat(self):
+        """Should explain improving predicate when radius is unchanged."""
+        explainer = PredicateExplainer(enabled=True, use_color=False)
+        stderr = StringIO()
+        with patch("sys.stderr", stderr):
+            explainer.explain_improving(
+                result=False,
+                previous_radius=5.0,
+                current_radius=5.0,
+                trajectory_length=3,
+            )
+        output = stderr.getvalue()
+        assert "`improving` → FALSE" in output
+        assert "unchanged (not improving)" in output
+
     def test_explain_improving_insufficient_data(self):
         """Should explain improving with insufficient trajectory."""
         explainer = PredicateExplainer(enabled=True, use_color=False)