TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 8: refactor(gui_2): migrate L591 _diag_layout_state to Result[T] (Phase 8)

Migrate the ini-file-read try/except in App._diag_layout_state (L591) to
the canonical Result[T] pattern:

- Extract _diag_layout_state_ini_text_result(app, ini_path) -> Result[str]
  helper into new Phase 8 Property Setter / State Result Helpers region.
- The legacy _diag_layout_state method calls the helper and drains errors
  to app._startup_timeline_errors (the Phase 2 drain plane for startup
  callbacks).
- The original fallback behavior (early return on read failure, stderr
  write for visibility) is preserved.

Tests:
- test_phase_8_l591_diag_layout_state_ini_text_result_success
- test_phase_8_l591_diag_layout_state_ini_text_result_failure

Audit: INTERNAL_BROAD_CATCH count in src/gui_2.py dropped from 2 to 1
(remaining: L896 _capture_workspace_profile, formerly L897 in inventory).

src/gui_2.py

@@ -584,13 +584,12 @@ class App:
    return
   ini_size = _os.path.getsize(ini_path)
   sys.stderr.write(f"[GUI] layout file: {ini_path} ({ini_size} bytes)\n")
-  #Note(Ed): Exception(Thirdparty)
-  try:
-   with open(ini_path, encoding="utf-8") as _f:
-    _ini_text = _f.read()
-  except Exception as _e:
-   sys.stderr.write(f"[GUI] could not read layout file: {_e}\n")
+  result = _diag_layout_state_ini_text_result(self, ini_path)
+  if not result.ok:
+   if not hasattr(self, "_startup_timeline_errors"): self._startup_timeline_errors = []
+   self._startup_timeline_errors.append(("_diag_layout_state", result.errors[0]))
    return
+  _ini_text = result.data
   _STALE_WINDOW_NAMES = {
    "Projects",      "Files",   "Screenshots", "Discussion History",
    "Provider",      "Message", "Response",    "Tool Calls",
@@ -8102,4 +8101,33 @@ def _worker_context_preview_result(app: "App") -> Result[None]:
 
 #endregion: Phase 7 Worker/Background Result Helpers
 
+#region: Phase 8 Property Setter / State Result Helpers (result_migration_gui_2_20260619)
+
+def _diag_layout_state_ini_text_result(app: "App", ini_path: str) -> Result[str]:
+ """Drain-aware variant of L591 _diag_layout_state ini-file-read try block.
+
+ Extracts the thirdparty file-open try/except from App._diag_layout_state
+ into a Result-returning helper. On exception, returns Result(data="",
+ errors=[ErrorInfo]) so the legacy wrapper can early-return. On success,
+ returns Result(data=ini_text) where ini_text is the file content. The
+ legacy wrapper drains errors to app._startup_timeline_errors (startup
+ callback drain plane).
+
+ [C: src/gui_2.py:App._diag_layout_state (L591 legacy wrapper)]
+ """
+ try:
+  with open(ini_path, encoding="utf-8") as _f:
+   ini_text = _f.read()
+  return Result(data=ini_text)
+ except Exception as e:
+  sys.stderr.write(f"[GUI] could not read layout file: {e}\n")
+  return Result(data="", errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Could not read layout file {ini_path}: {e}",
+   source="gui_2._diag_layout_state_ini_text_result",
+   original=e,
+  )])
+
+#endregion: Phase 8 Property Setter / State Result Helpers
+
 #endregion: MMA