fix: SRT 按 cue 解析并按 ID 回填译文，OCR 空帧分段与失败重试

tests/test_ocr_flow.py

@@ -397,8 +397,8 @@ def test_ocr_merges_consecutive_same_text(monkeypatch, tmp_path) -> None:
     assert "00:00:06,000 --> 00:00:10,000" in srt
 
 
-def test_ocr_skips_failed_frames(monkeypatch, tmp_path) -> None:
-    """个别帧 OCR 失败时跳过，不影响其余帧汇总。"""
+def test_ocr_preserves_checkpoint_for_failed_frames(monkeypatch, tmp_path) -> None:
+    """个别帧持续失败时返回 failed，其余成功帧存档供重试复用。"""
     frames = []
     for index in range(3):
         image = tmp_path / f"f{index}.png"
@@ -421,9 +421,11 @@ def test_ocr_skips_failed_frames(monkeypatch, tmp_path) -> None:
             output_dir=str(tmp_path / "out"),
         )
     )
-    assert response.status == "completed", response.error
-    srt = Path(response.outputs["srt_uri"]).read_text(encoding="utf-8")
-    assert "SUB 001" in srt
+    assert response.status == "failed"
+    partial = [json.loads(line) for line in (tmp_path / "out/ocr_partial.jsonl").read_text().splitlines()]
+    assert {item["frame"] for item in partial} == {0, 2}
+    assert all(item["text"] == "SUB 001" for item in partial)
+    assert not (tmp_path / "out/subtitle.srt").exists()
 
 
 def test_ocr_missing_manifest(tmp_path) -> None: