feat: add montage subcommand for multi-PNG grid stitching (Pillow)

tests/test_montage.py

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+"""montage_pngs 单元测试。"""
+
+import tempfile
+from pathlib import Path
+
+from PIL import Image
+
+
+def _make_test_png(path, w=200, h=100, color="red"):
+    """创建测试用 PNG。"""
+    img = Image.new("RGB", (w, h), color)
+    img.save(path)
+    return str(path)
+
+
+def test_montage_basic():
+    """4 张图拼成 2x2 网格。"""
+    from ts_anomaly_td.visualize import montage_pngs
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        paths = []
+        for i, color in enumerate(["red", "green", "blue", "yellow"]):
+            p = Path(tmpdir) / f"img_{i}.png"
+            _make_test_png(p, color=color)
+            paths.append(str(p))
+
+        out = Path(tmpdir) / "montage.png"
+        result = montage_pngs(paths, str(out), cols=2, title="Test")
+        assert Path(result).exists()
+        assert Path(result).stat().st_size > 1000
+
+
+def test_montage_uneven():
+    """3 张图拼成 2 列（第二行只 1 张）。"""
+    from ts_anomaly_td.visualize import montage_pngs
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        paths = []
+        for i in range(3):
+            p = Path(tmpdir) / f"img_{i}.png"
+            _make_test_png(p)
+            paths.append(str(p))
+
+        out = Path(tmpdir) / "montage_uneven.png"
+        result = montage_pngs(paths, str(out), cols=2)
+        assert Path(result).exists()
+
+
+def test_montage_empty():
+    """空列表应抛 ValueError。"""
+    from ts_anomaly_td.visualize import montage_pngs
+
+    try:
+        montage_pngs([], "/dev/null")
+        assert False, "应抛 ValueError"
+    except ValueError:
+        pass