desktop: 联调发现的透明窗口修复 + 联调验收报告与文档索引

- tauri.conf.json 启用 macOSPrivateApi、Cargo.toml 加 macos-private-api feature：
  overlay/tray 为 transparent 窗口，macOS 上 WKWebView 透明必须开启该项，
  否则浮层带不透明底色（tauri dev 启动日志明确告警）
- doc/desktop-integration-acceptance.html：#12/#1C 联调验收报告
  （mac 全链路通过、12B 延迟基线与短句问题、12C 四场景全过、商业链路通过、遗留项清单）
- doc/index.html：项目文档索引（全局文档规范要求）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

desktop/src-tauri/Cargo.toml

@@ -12,7 +12,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = ["tray-icon"] }
+tauri = { version = "2", features = ["tray-icon", "macos-private-api"] }
 tauri-plugin-global-shortcut = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"

desktop/src-tauri/tauri.conf.json

@@ -10,6 +10,7 @@
     "frontendDist": "../dist"
   },
   "app": {
+    "macOSPrivateApi": true,
     "windows": [
       {
         "label": "settings",