test: add monotonic window-size invariant (38/38)

Smaller window = less IDF contamination = more echo credit.
Fixed previous inconclusive test: old fixture had only 2 agents,
so IDF couldn't differentiate. New fixture uses 6 agents + silence gap.

test_swarmmetrics.py

@@ -891,3 +891,34 @@ def test_windowed_idf_prevents_semantic_hysteresis():
     # (windowed delta should be closer to zero than global delta)
     assert mean_windowed_delta >= mean_global_delta, \
         f"Windowed IDF should reduce suppression: windowed Δ={mean_windowed_delta:.3f} vs global Δ={mean_global_delta:.3f}"
+
+
+def test_windowed_idf_monotonic_with_window_size():
+    """Wider window = more contamination = less echo credit. Strictly monotonic."""
+    messages = []
+    # originator introduces alpha
+    messages.append({"from_id": "originator", "to_id": "_all",
+                     "timestamp": 0.0, "concepts": ["alpha"]})
+    # 4 agents echo alpha heavily in phase 1 (t=50-850)
+    for i in range(20):
+        for j, agent in enumerate(["flood1", "flood2", "flood3", "flood4"]):
+            messages.append({
+                "from_id": agent, "to_id": "_all",
+                "timestamp": float(50 + i * 40 + j * 10),
+                "concepts": ["alpha"]
+            })
+    # Phase 2 (t=2000): fresh agent echoes alpha after long silence
+    messages.append({"from_id": "fresh", "to_id": "_all",
+                     "timestamp": 2000.0, "concepts": ["alpha"]})
+
+    global_score = score_echo(messages, snapshot_idf=True)["originator"]
+
+    windows = [100, 300, 500, 1000, 2000]
+    prev = float("inf")
+    for w in windows:
+        s = score_echo(messages, snapshot_idf=True, idf_window_seconds=w)["originator"]
+        assert s >= global_score - 0.001, \
+            f"w={w}: windowed {s:.4f} < global {global_score:.4f}"
+        assert s <= prev + 0.001, \
+            f"w={w}: score {s:.4f} increased from previous {prev:.4f} (should decrease)"
+        prev = s