feat(export): 支持把原父目录名作为文件名前缀

新增 --prefix-parent：复制时把图片原来的父目录名拼到文件名前
（grease/abc.png -> grease_abc.png）。导出后目录只剩簇号，来源信息本来会丢失，
加前缀能留住它，人工核对时一眼看出某张图原本在哪个目录。

- 重名检查改为按**目标文件名**判重：加前缀后原本会撞车的文件可能就不撞了；
  不加前缀而撞车时，报错会提示可以试试这个开关
- manifest.json 记下 prefix_parent，导出的 file 字段也用实际写出的文件名

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

tests/test_export_clusters.py

@@ -4,6 +4,7 @@ import csv
 import filecmp
 import json
 import sys
+from pathlib import Path
 
 import pytest
 
@@ -78,6 +79,67 @@ def test_exports_real_files_with_original_names(cluster_run, tmp_path, monkeypat
         assert filecmp.cmp(original, by_name[original.name], shallow=False)
 
 
+def test_prefix_parent_prepends_source_folder(cluster_run, tmp_path, monkeypatch):
+    """--prefix-parent 把原父目录名拼到文件名前，导出后仍能看出图来自哪个目录。"""
+    _, work_dir, files = cluster_run
+    data_root = tmp_path / "data"
+
+    assert run(["--work-dir", str(work_dir), "--data-root", str(data_root), "--prefix-parent"],
+               monkeypatch) == 0
+
+    out = data_root / "film_r18_cluster_k2"
+    exported = {p.name for p in out.rglob("*.png")}
+    assert exported == {f"{f.parent.name}_{f.name}" for f in files}
+    assert all(name.startswith(("grease_", "wrinkle_")) for name in exported)
+
+    # 仍然是真实文件、内容不变
+    for path in out.rglob("*.png"):
+        assert not path.is_symlink() and path.is_file()
+
+    manifest = json.loads((out / "manifest.json").read_text("utf-8"))
+    assert manifest["prefix_parent"] is True
+    for entries in manifest["files"].values():
+        for entry in entries:
+            source = Path(entry["source"])
+            assert entry["file"] == f"{source.parent.name}_{source.name}"
+
+
+def test_without_prefix_names_are_unchanged(cluster_run, tmp_path, monkeypatch):
+    _, work_dir, files = cluster_run
+    data_root = tmp_path / "data"
+    run(["--work-dir", str(work_dir), "--data-root", str(data_root)], monkeypatch)
+
+    exported = {p.name for p in (data_root / "film_r18_cluster_k2").rglob("*.png")}
+    assert exported == {f.name for f in files}
+
+
+def test_prefix_parent_resolves_same_name_across_folders(cluster_run, tmp_path, monkeypatch):
+    """不同子目录的同名文件，不加前缀会撞车报错，加了前缀就能共存。"""
+    source, work_dir, _ = cluster_run
+    twin = source / "wrinkle" / "grease_0.png"      # 与 grease/grease_0.png 同名
+    Image.fromarray(np.zeros((20, 30, 3), dtype=np.uint8)).save(twin)
+
+    rows = list(csv.DictReader((work_dir / "assignments.csv").open(encoding="utf-8")))
+    with (work_dir / "assignments.csv").open("w", newline="", encoding="utf-8") as handle:
+        writer = csv.DictWriter(handle, fieldnames=list(rows[0]))
+        writer.writeheader()
+        writer.writerows(rows)
+        writer.writerow({**rows[0], "path": str(twin), "cluster": rows[0]["cluster"]})
+
+    data_root = tmp_path / "data"
+    argv = ["--work-dir", str(work_dir), "--data-root", str(data_root)]
+    assert run(argv, monkeypatch) == 1                       # 不加前缀：撞车
+    assert run([*argv, "--prefix-parent"], monkeypatch) == 0  # 加前缀：共存
+
+    out = data_root / "film_r18_cluster_k2"
+    assert {"grease_grease_0.png", "wrinkle_grease_0.png"} <= {p.name for p in out.rglob("*.png")}
+
+
+def test_destination_name_helper():
+    assert ec.destination_name(Path("data/raw/film/grease/abc.png")) == "abc.png"
+    assert ec.destination_name(Path("data/raw/film/grease/abc.png"), True) == "grease_abc.png"
+
+
 def test_cluster_dirs_are_numbered_from_zero(cluster_run, tmp_path, monkeypatch):
     _, work_dir, files = cluster_run
     data_root = tmp_path / "data"