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/Editor.cpp

@@ -17,6 +17,7 @@
 #include "StatsPanel.h"
 
 #include <Engine/Core/Log.h>
+#include <Engine/Core/Time.h>
 #include <Engine/Serialization/SceneSerializer.h>
 
 #include <imgui.h>
@@ -36,6 +37,8 @@ Editor::~Editor() = default;
 void Editor::Init(const EditorContext& ctx)
 {
     m_Ctx = ctx;
+    // PIE 运行态指针：指向本对象成员（ctx 拷贝后补接，面板只读）
+    m_Ctx.playState = &m_Playing;
 
     m_Hierarchy = std::make_unique<HierarchyPanel>();
     m_Inspector = std::make_unique<InspectorPanel>();
@@ -44,6 +47,60 @@ void Editor::Init(const EditorContext& ctx)
     m_AssetBrowser = std::make_unique<AssetBrowserPanel>();
 }
 
+// ---- PIE（Play-In-Editor）----
+
+// Play：当前场景 → 内存 YAML 快照 → 切游戏渲染（primary 相机实体）
+// → 解冻时间（物理/脚本/动画开始跑）。快照失败不进 Play（现场不动）
+void Editor::DoPlay()
+{
+    if (!m_Ctx.scene || !m_Ctx.assets || !m_Ctx.renderer) return;
+
+    m_PlaySnapshot = Serialization::SceneSerializer::SerializeToString(
+        *m_Ctx.scene, *m_Ctx.assets, "PlaySnapshot");
+    if (m_PlaySnapshot.empty())
+    {
+        ENGINE_ERROR("PIE: snapshot failed, play aborted");
+        return;
+    }
+
+    m_Playing = true;
+    m_Ctx.renderer->SetDisplayMode(kDisplayDirect);
+    Time::SetTimeScale(1.0f);
+    ENGINE_INFO("PIE: playing (scene snapshot {} bytes)",
+                m_PlaySnapshot.size());
+}
+
+// Stop：快照原子换血恢复 → 回编辑器渲染 → 冻结时间。
+// 换血后所有 Entity 句柄悬空：清选中（描边同步清）、Gizmo 拖拽态
+// 复位。物理 body/约束/角色由对账系统随实体重建/清理，无需干预。
+// 恢复失败（快照损坏，理论罕见）也必须回编辑态，现场留给用户
+// File→Load 或退出前 Save
+void Editor::DoStop()
+{
+    if (!m_Ctx.scene || !m_Ctx.assets || !m_Ctx.renderer) return;
+
+    std::string envPath;
+    if (!Serialization::SceneSerializer::DeserializeFromString(
+            *m_Ctx.scene, *m_Ctx.assets, m_PlaySnapshot, envPath))
+    {
+        ENGINE_ERROR("PIE: snapshot restore failed (scene is now the "
+                     "played-out state)");
+    }
+    m_PlaySnapshot.clear();
+
+    m_Playing = false;
+    m_Ctx.renderer->SetDisplayMode(kDisplayViewport);
+    Time::SetTimeScale(0.0f);
+    SelectEntity(entt::null);
+    m_Gizmo.dragging = false;
+    ENGINE_INFO("PIE: stopped, editor scene restored");
+}
+
+void Editor::TogglePlay()
+{
+    m_Playing ? DoStop() : DoPlay();
+}
+
 void Editor::Draw()
 {
     // ImGuizmo 每帧首调（官方约定）：内部会清 mbOverGizmoHotspot 等
@@ -133,8 +190,47 @@ void Editor::DrawMenuBar()
         return;
     }
 
+    // ---- PIE 工具栏（UE 主工具栏语义）：Play/Stop 切换按钮 +
+    // Ctrl+P 全局热键。运行态按钮变 Stop（黄底提示在视口内）----
+    {
+        const ImGuiIO& io = ImGui::GetIO();
+        // Ctrl+P：文本输入中（改名/路径框）不触发，防打字误切 PIE
+        if (!m_Playing && io.KeyCtrl && !io.WantTextInput &&
+            ImGui::IsKeyPressed(ImGuiKey_P, false))
+        {
+            TogglePlay();
+        }
+
+        if (m_Playing)
+        {
+            ImGui::PushStyleColor(ImGuiCol_Button,
+                                  ImVec4(0.75f, 0.45f, 0.10f, 1.0f));
+            ImGui::PushStyleColor(ImGuiCol_ButtonHovered,
+                                  ImVec4(0.85f, 0.55f, 0.15f, 1.0f));
+            ImGui::PushStyleColor(ImGuiCol_ButtonActive,
+                                  ImVec4(0.90f, 0.62f, 0.20f, 1.0f));
+            if (ImGui::SmallButton("Stop (Ctrl+P)"))
+            {
+                TogglePlay();
+            }
+            ImGui::PopStyleColor(3);
+            ImGui::SameLine();
+            ImGui::TextColored(ImVec4(1.0f, 0.72f, 0.25f, 1.0f),
+                               "PLAYING - editor input disabled");
+        }
+        else if (ImGui::SmallButton("Play (Ctrl+P)"))
+        {
+            TogglePlay();
+        }
+        ImGui::SameLine();
+        ImGui::Separator();
+    }
+
     if (ImGui::BeginMenu("File"))
     {
+        // 运行态禁存取：Save 会把"玩出来的状态"当编辑结果落盘，
+        // Load 会撕掉快照恢复链（UE 运行态同样禁）
+        ImGui::BeginDisabled(m_Playing);
         if (ImGui::MenuItem("Save Scene...", nullptr))
         {
             m_ShowSaveModal = true;
@@ -143,6 +239,7 @@ void Editor::DrawMenuBar()
         {
             m_ShowLoadModal = true;
         }
+        ImGui::EndDisabled();
         ImGui::Separator();
         if (ImGui::MenuItem("Quit", "Alt+F4"))
         {