feat(cluster): 输出簇分布图，直观展示簇间距离

在 preview 拼图的同级目录生成 cluster_scatter.png：
- 左：聚类空间的 PCA 二维投影，散点按簇着色 + 质心标记，标题标注两个主成分的方差占比
- 右：质心两两距离矩阵（在聚类空间里算，不是投影后），带数值标注

投影用 PCA 而不是 t-SNE/UMAP——后两者为了把簇拉开会扭曲簇间距离，
而这张图的目的就是看距离。

- ClusterResult 新增 points 字段，保留 KMeans 实际使用的空间供可视化复用，
  质心与散点用同一组主成分变换，避免质心飘到簇外
- summary.json 新增 centroid_distances
- 新增 scatter 配置块（enabled / max_points / point_size / alpha / figsize / dpi）
- 中文字体按 PingFang / Noto Sans CJK 等顺序探测，找不到自动退回英文标签，
  不会画成豆腐块
- 新增依赖 matplotlib

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_simclr.py

@@ -337,6 +337,50 @@ def test_cluster_script_simclr_branch_end_to_end(tiny_image_dir, tmp_path, monke
     assert (work_dir / "clusters" / "cluster_00").is_dir()
 
 
+def test_scatter_plot_is_written_next_to_previews(tiny_image_dir, tmp_path, monkeypatch):
+    """cluster_scatter.png 要和 preview 拼图落在同一级目录。"""
+    sys.path.insert(0, "scripts")
+    import cluster_unlabeled as cu
+
+    work_dir = tmp_path / "run"
+    monkeypatch.setattr(
+        sys, "argv",
+        ["cluster_unlabeled.py", "--config", "configs/cluster/film.yaml",
+         "--work-dir", str(work_dir), "--root", str(tiny_image_dir), "--set",
+         "device=cpu", "loader.num_workers=0", "loader.batch_size=4",
+         "num_clusters=2", "n_init=2"],
+    )
+    assert cu.main() == 0
+
+    scatter = work_dir / "cluster_scatter.png"
+    assert scatter.exists() and scatter.stat().st_size > 5000
+    assert (work_dir / "cluster_00_preview.png").parent == scatter.parent
+
+    import json
+
+    summary = json.loads((work_dir / "summary.json").read_text(encoding="utf-8"))
+    matrix = summary["centroid_distances"]
+    assert len(matrix) == 2 and matrix[0][0] == pytest.approx(0.0)
+    assert matrix[0][1] == pytest.approx(matrix[1][0])
+
+
+def test_scatter_can_be_disabled(tiny_image_dir, tmp_path, monkeypatch):
+    sys.path.insert(0, "scripts")
+    import cluster_unlabeled as cu
+
+    work_dir = tmp_path / "run_off"
+    monkeypatch.setattr(
+        sys, "argv",
+        ["cluster_unlabeled.py", "--config", "configs/cluster/film.yaml",
+         "--work-dir", str(work_dir), "--root", str(tiny_image_dir), "--set",
+         "device=cpu", "loader.num_workers=0", "loader.batch_size=4",
+         "num_clusters=2", "n_init=2", "scatter.enabled=False"],
+    )
+    assert cu.main() == 0
+    assert not (work_dir / "cluster_scatter.png").exists()
+    assert (work_dir / "cluster_00_preview.png").exists()
+
+
 def test_simclr_runs_on_the_resolved_device(tiny_image_dir, tmp_path, monkeypatch):
     """method=simclr 在本机自动选中的后端上（MPS / CUDA / CPU）都应能跑通。"""
     sys.path.insert(0, "scripts")