feat(studio-3d): 历史回放里的三维面板 —— 由「重算」通道喂它，并标注来源

- What: `VIEW_SHOWS_3D` 加入 `replay`；`app_window` 新增 `_shows_3d()`（可见性与布局共用一个判据）
  与 `_splitter_sizes_for()`（预设表里的 3D 槽是"二维布局"的描述，面板打开时从最宽的面板借
  `DEFAULT_3D_PANE_WIDTH=340`）；`ThreeDPanel` 新增第三个来源标签（实时采集 / 回放：录像不含三维场 /
  重算通道：由录像重算，非记录值）与 `set_source_kind()`；`ThreeDCoordinator.show_rest()` 把参考模型
  放回屏幕并停止主张测量值（重算停止时调用，否则最后一帧会留在屏幕上）；`Mesh3dPanel` 转发这两者，
  并新增"加载中"状态的修正：切换传感器再切回来时若该包已加载，直接显示已就绪的画面而不是一条永远
  不会被替换的"正在加载"。
- Why: 录像里没有三维场（raw_*.mp4 是原始相机图像，`depth.stored=false`，CSV 只有力值），所以纯回放只能
  显示参考模型；但「从视频重算力值」本来就把同一个运行路径指向录像的原始视频，而采集线程上的三维 sink
  早已注册——因此重算一开始面板就会收到真实的形变帧。这两件事必须由界面区分：一个是"没有测量"，
  一个是"当前算法对这段录像会说什么"，把后者当成记录值展示就是另一种 `display_illustrative`。
- Verification: apps/studio 全量通过（test_scene3d_view.py 32 项，含"回放里面板可见且标注正确""重算时
  标注切换""停止重算后清回参考模型""实时视图标为实时"；test_scene3d_coordinator.py 15 项，含 show_rest
  与"停止时在飞的帧不会随后被显示"）；ui_check 570 项 0 失败（新增"已安装包必须被加载并标注来源"）；
  locale_check 0 缺失；仓库内工程包经 validate_scene 全部 ok。
- Not in this change: 逐帧保存展开场/深度（记录格式变更，设计文档明确否掉）；重算通道本身（已存在，本次
  只是把它的帧接到面板上）；示意位移（仍需 SDK lift + 已确认尺度）。

apps/studio/src/orisys_studio/ui/mesh3d_panel.py

@@ -70,8 +70,12 @@ class Mesh3dPanel(QFrame):
         self._source: Optional[SceneSource] = None
         self._sensor_type = ""
         self._expected: Optional[ExpectedGeometry] = None
+        self._source_kind = "live"
         self._enabled = False
         self._loaded = False
+        #: The package whose scene is in the view, so switching sensor away and back
+        #: does not reload it (and does not pretend to be loading it either).
+        self._loaded_path: Optional[Path] = None
         self._reported: Optional[Diagnostic] = None
         self._pending_settings = None
         self._message_kind = _UNAVAILABLE
@@ -121,11 +125,22 @@ class Mesh3dPanel(QFrame):
             return
         self._source = find_package(self._sensor_type, override=package_override)
         if self._source is None:
+            self._loaded_path = None
             self._reported = no_asset_diagnostic(self._sensor_type, override=package_override)
             self._show_message(
                 _NO_ASSET, directory=str(self._reported.details["directory"])
             )
             return
+        if (
+            self._view is not None
+            and self._loaded_path is not None
+            and self._source.path == self._loaded_path
+        ):
+            # This scene is already loaded (the sensor was switched away and back):
+            # showing "loading…" again would leave a line nothing ever replaces, while
+            # the view behind it is ready.
+            self._stack.setCurrentWidget(self._view)
+            return
         self._show_message(_LOADING, name=self._source.path.name)
 
     def ensure_view(self) -> Optional[QWidget]:
@@ -141,6 +156,10 @@ class Mesh3dPanel(QFrame):
         view.enabledChanged.connect(self._on_view_enabled)
         if self._pending_settings is not None and hasattr(view, "restore_settings"):
             view.restore_settings(self._pending_settings)
+        # The kind was decided before the view existed (the window sets it when the
+        # view preset changes), so it is pushed again here rather than lost.
+        if hasattr(view, "set_source_kind"):
+            view.set_source_kind(self._source_kind)
         if not view.available:
             self._show_message(_UNAVAILABLE, reason=view.reason)
             return view
@@ -186,6 +205,7 @@ class Mesh3dPanel(QFrame):
             return
         if not view.use_scene(assets, source_id=description, expected=self._expected):
             return
+        self._loaded_path = None if self._source is None else self._source.path
         self._stack.setCurrentWidget(view)
         # Adopting a scene is not the same as showing one: the pane stays off until
         # the operator (or the window, restoring a preference) turns it on.
@@ -193,6 +213,7 @@ class Mesh3dPanel(QFrame):
 
     def _on_load_failed(self, item: Diagnostic) -> None:
         self._reported = item
+        self._loaded_path = None
         self._show_message(_REJECTED, message=item.message)
 
     def _on_diagnostic(self, item: Diagnostic) -> None:
@@ -239,6 +260,17 @@ class Mesh3dPanel(QFrame):
     def scene_path(self) -> Optional[Path]:
         return None if self._source is None else self._source.path
 
+    def set_source_kind(self, kind: str) -> None:
+        """Where the field comes from: live, replay (model only) or recompute."""
+        self._source_kind = str(kind)
+        if self._view is not None and hasattr(self._view, "set_source_kind"):
+            self._view.set_source_kind(self._source_kind)
+
+    def show_reference_only(self) -> None:
+        """Clear a measurement that is no longer being produced (a stopped pass)."""
+        if self._view is not None and hasattr(self._view, "show_reference_only"):
+            self._view.show_reference_only()
+
     def tick(self) -> None:
         """Advance the pane. Called by the window's run timer, never by itself."""
         if self._view is not None and hasattr(self._view, "tick"):