新建 UI 设计资源复用无冲突编号规则

- ui_editor/resource_bridge.rs：`next_ui_design_path` 提升为 `pub(crate)` 并补注释，作为两个写入侧唯一的「UI 设计 N」命名规则。
- commands.rs：`create_ui_design_resource` 不再按已登记文档数直接取名，改为复用该规则，resourceId 与文件名同一编号。
- tests/project.rs：新增已存在 `ui/UI 设计 1.json` 时仍能创建并落到编号 2 的回归用例。

apps/ai-game-creator-shell/src-tauri/src/commands.rs

@@ -2197,17 +2197,8 @@ pub(crate) fn create_ui_design_resource(
     if manifest.project_id != expected_project_id.trim() {
         return Err("project-identity-conflict".to_string());
     }
-    let next_index = manifest
-        .assets
-        .iter()
-        .filter(|asset| {
-            asset.kind == crate::ui_editor::persistence::UI_DESIGN_DOC_ASSET_KIND
-                && asset.media_type == crate::ui_editor::persistence::UI_DESIGN_DOC_MEDIA_TYPE
-        })
-        .count()
-        + 1;
-    let resource_name = format!("UI 设计 {next_index}");
-    let relative_path = format!("ui/{resource_name}.json");
+    let (resource_name, relative_path) =
+        crate::ui_editor::resource_bridge::next_ui_design_path(root, &manifest)?;
     let absolute_path = resolve_local_project_path(root, &relative_path)?;
     if let Some(parent) = absolute_path.parent() {
         ensure_game_creator_private_directory_tree(parent, "UI 资源目录")?;
@@ -2243,7 +2234,10 @@ pub(crate) fn create_ui_design_resource(
         GameCreationAppAssetSource {
             kind: GameCreationAppAssetSourceKind::Generated,
             canvas_project_id: None,
-            resource_id: Some(format!("ui:{next_index}")),
+            resource_id: Some(format!(
+                "ui:{}",
+                resource_name.trim_start_matches("UI 设计 ")
+            )),
             asset_object_id: None,
             task_id: None,
             prompt: None,

apps/ai-game-creator-shell/src-tauri/src/tests/project.rs

@@ -2259,6 +2259,37 @@ fn register_local_asset_derives_category_from_real_write_side_kinds() {
     fs::remove_dir_all(root).ok();
 }
 
+/// 已存在的 `ui/UI 设计 N.json` 不能让新建 UI 设计资源直接失败。
+///
+/// 编号按已登记的 UI 文档数推导，旧项目里 kind 已被收口为 `unknown` 的文档不再计入，
+/// 只按计数取名就会撞上仍然存在的同名文件并报「路径已存在」。两个写入侧必须共用
+/// `ui_editor::resource_bridge` 的「第一个空闲编号」规则，既不改名既有文件也不覆盖。
+#[test]
+fn create_ui_design_resource_skips_a_taken_ui_design_path() {
+    let root = unique_project_path();
+    init_local_game_project_at(&root, "project-1", "UI 设计编号避让").expect("project init");
+    write_local_project_file_at(&root, "ui/UI 设计 1.json", "{}").expect("ui asset file");
+
+    let result = crate::commands::create_ui_design_resource(
+        root.to_string_lossy().to_string(),
+        "project-1".to_string(),
+    )
+    .expect("create UI design resource");
+
+    assert_eq!(result.asset.local_path, "ui/UI 设计 2.json");
+    let registered = result
+        .manifest
+        .assets
+        .iter()
+        .find(|asset| asset.id == result.asset.id)
+        .expect("registered UI design asset");
+    assert_eq!(registered.source.resource_id.as_deref(), Some("ui:2"));
+    assert_eq!(registered.kind, GameCreationAppAssetKind::UiDesignDoc);
+    assert!(root.join("ui/UI 设计 1.json").is_file());
+
+    fs::remove_dir_all(root).ok();
+}
+
 #[test]
 fn register_local_asset_rejects_missing_or_unsafe_path() {
     let root = unique_project_path();

apps/ai-game-creator-shell/src-tauri/src/ui_editor/resource_bridge.rs

@@ -190,7 +190,9 @@ pub(crate) fn ensure_ui_design_resource_for_prototype(
     })
 }
 
-fn next_ui_design_path(
+/// UI 设计文档的确定性命名：从已登记文档数 + 1 起找第一个既未被占用、
+/// 也未登记进 manifest 的 `ui/UI 设计 N.json`，不覆盖任何既有文件。
+pub(crate) fn next_ui_design_path(
     root: &Path,
     manifest: &GameCreationAppManifest,
 ) -> Result<(String, String), String> {