fix(editor): asset browser directory navigation

The file list was hardwired to the root directory, so it was always
empty (all assets live in subdirectories). Directories are now
selectable from both the tree (click row) and the list (click [D]
row), with a breadcrumb + Root button. Prefix check done manually
(path::starts_with is missing in this libstdc++).

App/editor/AssetBrowserPanel.cpp

@@ -111,15 +111,50 @@ void AssetBrowserPanel::Draw(const EditorContext& ctx)
     ImGui::Separator();
 
     const std::filesystem::path root(kRootPaths[m_Root]);
+    // 根切换 / 首帧：当前目录回根。前缀比较手写（path::starts_with
+    // 是 libstdc++ 11 才补齐的成员，本机 GCC 没有）；补 '/' 分隔校验
+    // 防 "assets_x" 之类误匹配
+    const std::string rootStr = root.generic_string();
+    const std::string curStr = m_CurrentDir.generic_string();
+    const bool under = curStr == rootStr ||
+                       (curStr.size() > rootStr.size() &&
+                        curStr.compare(0, rootStr.size(), rootStr) == 0 &&
+                        curStr[rootStr.size()] == '/');
+    if (m_CurrentDir.empty() || !under)
+    {
+        m_CurrentDir = root;
+    }
 
-    // ---- 左：目录树 | 右：文件列表 ----
+    // ---- 顶栏下：面包屑（当前目录相对根路径 + 回根按钮）----
+    if (ImGui::Button("Root"))
+    {
+        m_CurrentDir = root;
+    }
+    ImGui::SameLine();
+    std::string rel = std::filesystem::relative(m_CurrentDir, root)
+                          .generic_string();
+    ImGui::TextDisabled("%s/%s", kRootNames[m_Root],
+                        rel.empty() ? "" : rel.c_str());
+    ImGui::Separator();
+
+    // ---- 左：目录树 | 右：当前目录文件列表 ----
     const float treeWidth = 180.0f;
     ImGui::BeginChild("##tree", ImVec2(treeWidth, 0.0f),
                       ImGuiChildFlags_ResizeX | ImGuiChildFlags_Borders);
     if (std::filesystem::exists(root))
     {
+        // 根节点也画进树（可点击选中）
         ImGui::SetNextItemOpen(true, ImGuiCond_Once);
-        DrawDirTree(ctx, root);
+        if (ImGui::TreeNodeEx(kRootNames[m_Root],
+                              ImGuiTreeNodeFlags_OpenOnArrow))
+        {
+            if (ImGui::IsItemClicked(ImGuiMouseButton_Left))
+            {
+                m_CurrentDir = root;
+            }
+            DrawDirTree(ctx, root);
+            ImGui::TreePop();
+        }
     }
     else
     {
@@ -130,9 +165,9 @@ void AssetBrowserPanel::Draw(const EditorContext& ctx)
     ImGui::SameLine();
     ImGui::BeginChild("##files", ImVec2(0.0f, 0.0f),
                       ImGuiChildFlags_Borders);
-    if (std::filesystem::exists(root))
+    if (std::filesystem::exists(m_CurrentDir))
     {
-        DrawFileList(ctx, root);
+        DrawFileList(ctx, m_CurrentDir);
     }
     ImGui::EndChild();
 
@@ -149,7 +184,15 @@ void AssetBrowserPanel::DrawDirTree(const EditorContext& ctx,
             continue;
         }
         const std::string name = entry.path().filename().string();
-        if (ImGui::TreeNode(name.c_str()))
+        // OpenOnArrow：单击行 = 选中（列表切换），箭头 = 折叠
+        const bool open = ImGui::TreeNodeEx(
+            name.c_str(), ImGuiTreeNodeFlags_OpenOnArrow |
+                              ImGuiTreeNodeFlags_SpanFullWidth);
+        if (ImGui::IsItemClicked(ImGuiMouseButton_Left))
+        {
+            m_CurrentDir = entry.path();
+        }
+        if (open)
         {
             DrawDirTree(ctx, entry.path());
             ImGui::TreePop();
@@ -172,6 +215,16 @@ void AssetBrowserPanel::DrawFileList(const EditorContext& ctx,
     {
         if (entry.is_directory())
         {
+            // 子目录也进列表：单行点击进入（树/列表双向导航）
+            const std::string dname = entry.path().filename().string();
+            ImGui::PushID(dname.c_str());
+            ImGui::TextUnformatted("[D]");
+            ImGui::SameLine();
+            if (ImGui::Selectable(dname.c_str()))
+            {
+                m_CurrentDir = entry.path();
+            }
+            ImGui::PopID();
             continue;
         }
         const Kind kind = KindOf(entry.path());