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。

apps/studio/tools/ui_check.py

@@ -3001,9 +3001,9 @@ def main() -> int:
         f"{replay_window.toolbar.source_summary.text()!r}",
     )
 
-    # A video that is not a recording must be refused *with a reason*, because the
-    # reader tolerates an empty folder: accepting it left the transport playing a
-    # zero-length timeline, which looked like "pressing 回放 does nothing".
+    # A video with no recording beside it is still something to *watch*: the transport
+    # plays it, the charts stay empty, and the rail/tooltip say why. Refusing it (the
+    # older behaviour) left an operator pointing at a clip with a dead button.
     import cv2 as _cv2
     import numpy as _np
 
@@ -3011,8 +3011,8 @@ def main() -> int:
     bare_writer = _cv2.VideoWriter(
         str(bare_video), _cv2.VideoWriter_fourcc(*"mp4v"), 30.0, (64, 48)
     )
-    for _index in range(10):
-        bare_writer.write(_np.zeros((48, 64, 3), dtype=_np.uint8))
+    for _index in range(30):
+        bare_writer.write(_np.full((48, 64, 3), _index * 8, dtype=_np.uint8))
     bare_writer.release()
     other_window = StudioMainWindow(
         prefs={"language": "zh_CN", "theme": "dark", "sensor_type": "box"}
@@ -3023,34 +3023,55 @@ def main() -> int:
     other_window.brand_bar.btn_playback.click()
     app.processEvents()
     check(
-        other_window._load_replay_video(bare_video) is False,
-        "a video with no forces.csv beside it was accepted as a recording",
+        other_window._load_replay_video(bare_video) is True,
+        "a playable video with no recording beside it was refused",
     )
     app.processEvents()
+    check(
+        other_window.replay.has_video() and other_window.replay.is_video_only(),
+        "the video is not playable as a video",
+    )
     check(
         not other_window.replay.is_loaded(),
-        "an empty folder was opened as a recording",
+        "a video with no forces.csv claimed to be a recording",
     )
     check(
-        not other_window.toolbar.btn_run.isEnabled(),
-        "the transport is offered although there is nothing to play",
+        other_window.toolbar.btn_run.isEnabled(),
+        "the transport is not offered for a video that can be played",
     )
     check(
-        "forces.csv" in other_window.toolbar.btn_run.toolTip()
-        or "记录" in other_window.toolbar.btn_run.toolTip(),
-        f"the transport does not say why it cannot play: "
+        t("toolbar.playback_video_only") in other_window.toolbar.btn_run.toolTip(),
+        f"the transport does not say the charts are empty: "
         f"{other_window.toolbar.btn_run.toolTip()!r}",
     )
+    check(
+        t("playback.video_only_notice") in other_window.replay.warnings_text(),
+        f"the video is not described as one: {other_window.replay.warnings_text()!r}",
+    )
+    check(
+        not other_window.force_plots.is_recording_source(),
+        "the charts claim to read a recording that does not exist",
+    )
+    other_window.toolbar.btn_run.setChecked(True)
+    app.processEvents()
+    _deadline = time.time() + 3.0
+    while time.time() < _deadline and other_window.replay.position() < 0.4:
+        app.processEvents()
+        time.sleep(0.05)
+    check(
+        other_window.replay.position() >= 0.4,
+        f"the transport did not play the video: {other_window.replay.position()}",
+    )
+    check(
+        other_window.replay.video_image() is not None,
+        "no frame was decoded from the video",
+    )
     check(
         other_window.act_recompute.isEnabled(),
         "a chosen video cannot be recomputed either: the operator has no way to view it",
     )
-    check(
-        "forces.csv" in other_window.arrow_panel.image_label.text()
-        or "记录" in other_window.arrow_panel.image_label.text(),
-        f"the panel does not say what is missing: "
-        f"{other_window.arrow_panel.image_label.text()!r}",
-    )
+    other_window._stop_replay_activity()
+    app.processEvents()
     other_window.close()
     app.processEvents()
     replay_window.activateWindow()