fix(gui): wire _install_default_layout_if_empty_result into App._post_init

App._post_init now resolves src = paths.get_layouts_dir()/default.ini
and dst = Path.cwd()/manualslop_layout.ini, then calls the drain-plane
helper before the warmup-complete registration block. Errors drain to
self._startup_timeline_errors per the data-oriented convention, so a
missing bundled layout (e.g. partial wheel install) does not crash the
GUI: panels just stay invisible until the user drops a real INI in.

Test fix: test_default_layout_install._GUI_SCRIPT was a relative path,
but the subprocess Popen runs with cwd = temp_workspace where sloppy.py
does not exist. Switched to an absolute path via _PROJECT_ROOT, the
same pattern conftest.py:648 uses for the live_gui fixture.

src/gui_2.py

@@ -574,6 +574,12 @@ class App:
   # _io_pool thread; it only sets primitive state on the App, which
   # is safe. The render_warmup_status_indicator() function reads
   # the timestamp to show a brief "ready" tag for 3 seconds.
+  src_layout_path: Path = paths.get_layouts_dir() / "default.ini"
+  dst_layout_path: Path = Path.cwd() / "manualslop_layout.ini"
+  install_result: Result[bool] = _install_default_layout_if_empty_result(self, src_layout_path, dst_layout_path)
+  if not install_result.ok:
+   if not hasattr(self, "_startup_timeline_errors"): self._startup_timeline_errors = []
+   self._startup_timeline_errors.append(("_install_default_layout", install_result.errors[0]))
   if hasattr(self.controller, "on_warmup_complete"):
    cb_result = _post_init_callback_result(self)
    if not cb_result.ok: