feat(gui): add _install_default_layout_if_empty helpers for install-on-empty-INI

Module-level _install_default_layout_if_empty(src, dst) reads the
bundled layout from src, decides if dst is missing/empty/small
(< 1000 bytes or no [Window][ header), copies src -> dst on true,
and returns Result[bool]. On OSError reading/writing, returns
Result[data=False, errors=[ErrorInfo]] so App._post_init can drain
to _startup_timeline_errors per the data-oriented convention.

_install_default_layout_if_empty_result(app, src, dst) is the
drain-plane passthrough that mirrors _post_init_callback_result.

Wiring into App._post_init lands in the next commit.

src/gui_2.py

@@ -1466,7 +1466,58 @@ def _post_init_callback_result(app: "App") -> Result[None]:
    message=f"on_warmup_complete callback registration failed: {e}",
    source="gui_2._post_init_callback_result",
    original=e,
+)])
+def _install_default_layout_if_empty(src_ini: Path, dst_ini: Path) -> Result[bool]:
+ """Install bundled layout to cwd when the user's INI is missing/empty/small.
+
+ Decision rule: dst_ini is "empty" when its content is fewer than 1000
+ bytes OR has no [Window][ header. On empty, copies src_ini -> dst_ini
+ and returns Result(data=True). On non-empty (user customized), returns
+ Result(data=False) without overwriting. On OSError reading src or
+ writing dst, returns Result(data=False, errors=[ErrorInfo]) so the
+ legacy wrapper in App._post_init can drain to _startup_timeline_errors.
+
+ [C: src/gui_2.py:_install_default_layout_if_empty_result,
+     src/gui_2.py:App._post_init]"""
+ try:
+  dst_text: str = dst_ini.read_text(encoding="utf-8", errors="replace") if dst_ini.exists() else ""
+ except OSError:
+  dst_text = ""
+ is_dst_empty: bool = len(dst_text) < 1000 or "[Window][" not in dst_text
+ if not is_dst_empty:
+  return Result(data=False)
+ try:
+  src_text: str = src_ini.read_text(encoding="utf-8", errors="replace")
+ except OSError as e:
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Could not read bundled layout {src_ini}: {e}",
+   source="gui_2._install_default_layout_if_empty",
+   original=e,
   )])
+ try:
+  dst_ini.parent.mkdir(parents=True, exist_ok=True)
+  dst_ini.write_text(src_text, encoding="utf-8")
+ except OSError as e:
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Could not write layout {dst_ini}: {e}",
+   source="gui_2._install_default_layout_if_empty",
+   original=e,
+  )])
+ sys.stderr.write(f"[GUI] installed default layout: {src_ini} -> {dst_ini}\n")
+ return Result(data=True)
+def _install_default_layout_if_empty_result(app: "App", src: Path, dst: Path) -> Result[bool]:
+ """Drain-aware variant of _install_default_layout_if_empty.
+
+ Passthrough wrapper so App._post_init can call the install via the
+ drain-plane naming convention (mirrors _post_init_callback_result).
+ The wrapped function already returns Result[bool] and catches OSError
+ internally; this wrapper exists for naming consistency and to give
+ any future exception-logging policy a single hook point.
+
+ [C: src/gui_2.py:_install_default_layout_if_empty, src/gui_2.py:App._post_init]"""
+ return _install_default_layout_if_empty(src, dst)
 def _run_immapp_result(app: "App") -> Result[None]:
  """Drain-aware variant of App.run immapp.run() call (L728 INTERNAL_SILENT_SWALLOW).