feat: default export dir to <install root>\合并后的图片 when packaged

get_output_dir() now returns <install root>\合并后的图片 (parent of app\) for
frozen builds — next to Launcher.exe, easy to find, not replaced on update.
Falls back to <data_dir>/output if the install root is read-only; dev mode
unchanged. The export panel already uses get_output_dir(), so its default
follows automatically.

- tests/test_file_service.py: +3 (dev, frozen install-root, fallback)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/services/file_service.py

@@ -79,8 +79,36 @@ def get_log_dir():
     return d
 
 
+def _is_dir_writable(path):
+    """Create *path* and probe it; True if a file can be written there."""
+    try:
+        path.mkdir(parents=True, exist_ok=True)
+        probe = path / ".write_test"
+        probe.write_text("x", encoding="ascii")
+        probe.unlink()
+        return True
+    except OSError:
+        return False
+
+
+# Default export folder name, placed next to Launcher.exe in the packaged layout.
+_OUTPUT_DIR_NAME = "合并后的图片"
+
+
 def get_output_dir():
-    """Return <data_dir>/output/ as a Path, creating the directory if absent."""
+    """Return the default export directory as a Path, creating it if absent.
+
+    Packaged: <install root>\\合并后的图片 (next to Launcher.exe — easy to find
+    and NOT replaced on update, unlike app\\). Install root is the parent of the
+    app\\ folder (get_app_dir()). Falls back to <data_dir>/output if the install
+    root is not writable. Development: <data_dir>/output (the project dir).
+
+    This is only the default; the user's chosen output_dir takes precedence.
+    """
+    if getattr(sys, "frozen", False):
+        candidate = get_app_dir().parent / _OUTPUT_DIR_NAME
+        if _is_dir_writable(candidate):
+            return candidate
     d = get_data_dir() / "output"
     d.mkdir(parents=True, exist_ok=True)
     return d

tasks.md

@@ -1002,8 +1002,8 @@
 任务：
 
 - [x] 文档：`docs/10` §4/§5、`docs/02` 配置说明、`docs/07` §7.5 对齐
-- [ ] `file_service.get_output_dir()`：打包态优先 `<安装根>\合并后的图片`（安装根 = `get_app_dir().parent`），不可写回退 `get_data_dir()/output`；开发态用项目目录；补单测
-- [ ] 导出面板默认值随之变化（沿用 `get_output_dir()`，无需单独改）
+- [x] `file_service.get_output_dir()`：打包态优先 `<安装根>\合并后的图片`（安装根 = `get_app_dir().parent`），不可写回退 `get_data_dir()/output`；开发态用项目目录；`tests/test_file_service.py` 3 个单测
+- [x] 导出面板默认值随之变化（沿用 `get_output_dir()`，无需单独改）
 - [ ] GUI 实测：打包运行后默认导出到 `<安装根>\合并后的图片`，且更新后仍在
 
 ## 18. 后续暂缓任务

tests/test_file_service.py

@@ -52,5 +52,56 @@ class TestGetDataDir(unittest.TestCase):
         self.assertEqual(fs.get_data_dir(), fs.get_app_dir())
 
 
+class TestGetOutputDir(unittest.TestCase):
+    """get_output_dir() default location (docs/10-lan-update.md §5)."""
+
+    def setUp(self):
+        import tempfile
+        self.tmp = Path(tempfile.mkdtemp())
+        self._env = os.environ.get("CMBOT_DATA_DIR")
+        self._frozen = getattr(sys, "frozen", None)
+        os.environ["CMBOT_DATA_DIR"] = str(self.tmp / "data")
+        if hasattr(sys, "frozen"):
+            del sys.frozen
+        self._orig_app_dir = fs.get_app_dir
+
+    def tearDown(self):
+        import shutil
+        fs.get_app_dir = self._orig_app_dir
+        if self._env is None:
+            os.environ.pop("CMBOT_DATA_DIR", None)
+        else:
+            os.environ["CMBOT_DATA_DIR"] = self._env
+        if self._frozen is None:
+            if hasattr(sys, "frozen"):
+                del sys.frozen
+        else:
+            sys.frozen = self._frozen
+        shutil.rmtree(str(self.tmp), ignore_errors=True)
+
+    def test_dev_uses_data_output(self):
+        # not frozen -> <data_dir>/output
+        self.assertEqual(fs.get_output_dir(), Path(os.environ["CMBOT_DATA_DIR"]) / "output")
+
+    def test_frozen_uses_install_root(self):
+        app = self.tmp / "install" / "app"
+        app.mkdir(parents=True)
+        fs.get_app_dir = lambda: app
+        sys.frozen = True
+        self.assertEqual(fs.get_output_dir(), self.tmp / "install" / "合并后的图片")
+
+    def test_frozen_falls_back_when_not_writable(self):
+        app = self.tmp / "install" / "app"
+        app.mkdir(parents=True)
+        fs.get_app_dir = lambda: app
+        sys.frozen = True
+        orig = fs._is_dir_writable
+        fs._is_dir_writable = lambda p: False
+        try:
+            self.assertEqual(fs.get_output_dir(), Path(os.environ["CMBOT_DATA_DIR"]) / "output")
+        finally:
+            fs._is_dir_writable = orig
+
+
 if __name__ == "__main__":
     unittest.main()