feat(studio): 回放可以直接播放「一条视频」——不必先按「从视频重算」

- What: 新增 `record/playback.py::VideoOnlySource`（一个视频文件的时间轴：时长来自
  `CAP_PROP_FRAME_COUNT/FPS`，按位置解码、顺序前进时只 step 不 seek、越界钳到最后一帧；没有帧计数或帧率就
  拒绝而不是猜）；`ReplayController` 增加 `open_video()`/`has_video()`/`is_video_only()`/`video_path()`，
  `duration_s`/`set_position`/`_refresh_video`/`close` 都覆盖这条路径，并在 `warnings_text()` 里说明
  "这段视频没有配套记录"；`app_window._load_replay_video()` 改为按"这条视频是什么"处理：
  同目录有 `forces.csv` → 当记录打开（原行为），否则**当视频播放**（曲线为空、明确提示），只有连解码都不行
  才拒绝并给出 `从视频重算` 的出路；transport/`_render_replay_frame`/`_replay_owns_image`/状态栏 tooltip
  一律改用 `has_video()`（而查曲线的 `_apply_replay_source` 仍用 `is_loaded()`）。
- Why: 用户要求"只导入视频也能用回放按钮"。此前有录像必须走「从视频重算」才能动，因为一条裸视频被当成
  "不是记录"而拒绝（更早是当成空记录"成功"打开→零长度时间轴→按回放没反应）。这两种都不对：视频就是可以看的
  东西，只是没有力值；把它按视频播放、把"没有记录"如实写在标签与提示里，才既不骗人也不挡人。
- Verification: `shared/tests/test_playback.py` +3（VideoOnlySource：时长/按位置取帧/顺序前进/越界钳位/关闭后
  返回空/不可解码与无时间轴被拒；顺带修回我在插入新类时弄丢的 `list_recordings` 的 `base = Path(root)`）；
  `test_ui_smoke.py` 改 1 增 1（裸视频可播放：`is_video_only`、transport 可用、tooltip 说明曲线为空、
  按下后位置推进且解出画面、停止有效；不可解码文件仍被拒并提示）；ui_check 587 项 0 失败（回放段改为验证
  "裸视频能播"）；apps/studio 与 shared 全量通过；locale_check 0 缺失。
- Not in this change: 裸视频的曲线（没有 CSV 就没有力值，这是事实而不是待办）；裸视频的三维形变（要形变得走
  「从视频重算」，面板标签在回放里仍然是"录像不含三维场"）；多段视频拼接播放。

shared/tests/test_playback.py

@@ -22,6 +22,7 @@ from orisys_app_shared.record import (
     ExperimentRecorder,
     RecordConfig,
     RecordingReader,
+    VideoOnlySource,
     list_recordings,
 )
 
@@ -235,6 +236,84 @@ def test_list_recordings_finds_directories_newest_first(recording):
     assert recording in found
 
 
+# ------------------------------------------------------------- a video on its own
+def _write_clip(path: Path, frames: int = 60, fps: float = 30.0) -> Path:
+    """A tiny real video, so the decode path is exercised rather than mocked."""
+    import cv2
+
+    writer = cv2.VideoWriter(
+        str(path), cv2.VideoWriter_fourcc(*"mp4v"), fps, (64, 48)
+    )
+    assert writer.isOpened(), "this build cannot write mp4v"
+    for index in range(frames):
+        writer.write(np.full((48, 64, 3), index * 4, dtype=np.uint8))
+    writer.release()
+    return path
+
+
+def test_a_video_with_no_recording_is_playable(tmp_path):
+    """A clip with nothing beside it answers the same questions a recording does."""
+    clip = _write_clip(tmp_path / "clip.mp4")
+    source = VideoOnlySource(clip)
+    try:
+        assert source.duration_s == pytest.approx(2.0, abs=0.2)
+        first, extra = source.video_frame_at(0.0)
+        later, _ = source.video_frame_at(1.5)
+        assert first is not None and later is not None
+        assert first.shape == (48, 64, 3)
+        assert float(later.mean()) > float(first.mean()), (
+            "the clip gets brighter over time; the same frame came back twice"
+        )
+        # Stepping forward is the playback case and must not need a seek.
+        stepped, _ = source.video_frame_at(1.5 + 1.0 / 30.0)
+        assert stepped is not None
+        # Past the end clamps to the last frame instead of failing.
+        assert source.video_frame_at(99.0)[0] is not None
+        # There is deliberately no frame *record*: a video has no measurements.
+        assert extra is None
+    finally:
+        source.close()
+    assert source.video_frame_at(0.0) == (None, None), "closed source still served frames"
+
+
+def test_a_file_that_is_not_a_video_is_refused(tmp_path):
+    broken = tmp_path / "broken.mp4"
+    broken.write_text("not a video at all", encoding="utf-8")
+    with pytest.raises(ValueError):
+        VideoOnlySource(broken)
+    with pytest.raises(FileNotFoundError):
+        VideoOnlySource(tmp_path / "missing.mp4")
+
+
+def test_a_video_without_a_timeline_is_refused(tmp_path, monkeypatch):
+    """No frame count or rate means no timeline, and a guessed one would lie.
+
+    The input is a decoder that reports neither, rather than a hand-made file: this
+    build will not even write a video with a zero frame rate.
+    """
+    import cv2
+
+    clip = _write_clip(tmp_path / "clip.mp4")
+    real_capture = cv2.VideoCapture
+
+    class NoTimeline:
+        def __init__(self, *args, **kwargs) -> None:
+            self._real = real_capture(*args, **kwargs)
+
+        def isOpened(self) -> bool:  # noqa: N802 - OpenCV spelling
+            return True
+
+        def get(self, prop):  # noqa: ANN001 - OpenCV spelling
+            return 0.0
+
+        def release(self) -> None:
+            self._real.release()
+
+    monkeypatch.setattr(cv2, "VideoCapture", NoTimeline)
+    with pytest.raises(ValueError):
+        VideoOnlySource(clip)
+
+
 if __name__ == "__main__":  # pragma: no cover - manual runner
     import shutil as _shutil