feat: Play-In-Editor (PIE) with in-memory scene snapshot

Editor toolbar gains Play/Stop (Ctrl+P) with UE semantics:

- Play: serialize current scene to an in-memory YAML snapshot via new
  SceneSerializer::SerializeToString, switch display to kDisplayDirect
  (primary camera entity renders), unfreeze time (timeScale 0 -> 1)
- Stop: DeserializeFromString atomically rebuilds the pre-play scene,
  back to kDisplayViewport (editor camera), refreeze time; selection
  cleared and gizmo drag reset since all entity handles dangle
- SceneSerializer refactored: SerializeDoc/ParseRoot shared by file
  and memory variants; file Save/Load behavior unchanged
- Running state disables gizmo + editor hotkeys in the viewport (input
  goes to game scripts), draws yellow PLAYING hint and a no-camera
  warning; File->Save/Load disabled while playing (UE behavior)

Physics bodies/constraints/characters and script instances are
reconciled automatically by the existing per-frame sync on restore.

App/editor/ViewportPanel.cpp

@@ -152,7 +152,10 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
                     selected != entt::null
                         ? reg.try_get<ECS::TransformComponent>(selected)
                         : nullptr;
-            if (out.focused && !ImGui::GetIO().WantTextInput &&
+            // PIE 运行态：输入归游戏（脚本读 GLFW 轮询），Gizmo/编辑器
+            // 热键全部让位（UE Play 语义：Scene view 编辑操作冻结）
+            const bool playing = ctx.playState && *ctx.playState;
+            if (!playing && out.focused && !ImGui::GetIO().WantTextInput &&
                 !ImGui::IsMouseDown(ImGuiMouseButton_Right)) {
                 if (ImGui::IsKeyPressed(ImGuiKey_W)) gizmo.op = ImGuizmo::TRANSLATE;
                 if (ImGui::IsKeyPressed(ImGuiKey_E)) gizmo.op = ImGuizmo::ROTATE;
@@ -169,7 +172,8 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
             // ---- Gizmo 操纵杆（ImGui drawlist 叠加，无 GPU pass）----
             // 选中实体 + 有 Transform 才出杆；矩阵分解直接写回
             // TransformComponent（与 Inspector 同源，下一帧数值自动跟随）
-            if (tr != nullptr) {
+            // PIE 运行态不出杆（编辑操作冻结）
+            if (tr != nullptr && !playing) {
                 // 相机矩阵：跟随渲染源（编辑器形态 = EditorCamera，游戏
                 // 形态 = primary 相机实体；与 SceneRenderer::BuildFrame
                 // 同一分派），参数与渲染侧一致（aspect = 渲染分辨率、
@@ -277,14 +281,23 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
             }
 
             // Gizmo 状态提示（左上角 drawlist 叠加，原因同下：不占布局行）
+            // PIE 运行态 = 黄色 PLAYING 提示替代编辑状态
             {
                 char tip[64];
-                std::snprintf(tip, sizeof(tip), "Space: %s [T]   Snap: Ctrl",
-                              gizmo.localSpace ? "Local" : "World");
+                if (playing) {
+                    std::snprintf(tip, sizeof(tip),
+                                  "PLAYING - Ctrl+P to stop");
+                } else {
+                    std::snprintf(tip, sizeof(tip),
+                                  "Space: %s [T]   Snap: Ctrl",
+                                  gizmo.localSpace ? "Local" : "World");
+                }
                 const ImVec2 imgMin = ImGui::GetItemRectMin();
                 ImGui::GetWindowDrawList()->AddText(
                         ImVec2(imgMin.x + 8.0f, imgMin.y + 8.0f),
-                        IM_COL32(255, 255, 255, 150), tip);
+                        playing ? IM_COL32(255, 184, 46, 220)
+                                : IM_COL32(255, 255, 255, 150),
+                        tip);
             }
 
             // Dynamic 刚体提示：物理每帧把姿态写回 Transform，拖拽即刻
@@ -292,6 +305,22 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
             // 用 drawlist 叠加而非 ImGui::TextDisabled：后者在 Image 下方
             // 占一行布局 → 滚动条出现/消失循环 → avail 抖动 → 渲染分辨率
             // 每帧重报（视口疯狂顶动）；且默认字体无中文字形（显示 ?）
+            if (playing) {
+                // 运行态无 primary 相机实体 = 游戏画面黑屏（BuildFrame
+                // 跳帧），提示补相机（Hierarchy 右键或 Add Component）
+                ECS::TransformComponent *camT = nullptr;
+                if (ViewCamera(ctx, camT) == nullptr) {
+                    const ImVec2 imgMin = ImGui::GetItemRectMin();
+                    ImGui::GetWindowDrawList()->AddText(
+                            ImVec2(imgMin.x + 8.0f,
+                                   imgMin.y + size.y * 0.5f),
+                            IM_COL32(255, 120, 80, 230),
+                            "No primary camera entity - add a Camera "
+                            "component and check 'Primary'");
+                }
+            }
+
+            // Dynamic 刚体提示（drawlist 叠加，理由同上）
             if (selected != entt::null) {
                 if (const auto *rb = reg.try_get<ECS::RigidBodyComponent>(selected);
                         rb != nullptr &&