refactor(app_controller): migrate 3 GUI state-setter sites to Result (Phase 6 Group 6.3)

Replaces logging.debug bodies in:
- _update_inject_preview (L1542): Result[str] variant; legacy wrapper
  stores error on self._inject_preview_error
- mcp_config_json setter (L1685): sibling _set_mcp_config_json_result
  helper (property setters can't return values); setter stores error
  on self._mcp_config_parse_error
- _save_active_project (L3124): Result[None] variant; legacy wrapper
  stores error on self._save_project_error and updates self.ai_status

Each error-carrying state attribute is the durable data plane for
sub-track 4 GUI to display; stderr write is the visible-but-incomplete
drain (full drain = GUI modal in sub-track 4).

Audit: INTERNAL_SILENT_SWALLOW for src/app_controller.py: 26 -> 23.

src/app_controller.py

@@ -837,6 +837,14 @@ class AppController:
   # sub-track 4 lands GUI-side error display); the instance list is
   # the durable data plane for sub-track 4 to drain.
   self._startup_timeline_errors:        List[Tuple[str, ErrorInfo]] = []  # (op_name, error)
+  # --- GUI state-setter error state (Phase 6: Group 6.3) ---
+  # Each carries the first ErrorInfo when the corresponding setter/operation
+  # fails. The legacy wrapper stores the error here for sub-track 4 GUI to
+  # display; stderr write IS the visible-but-incomplete drain (full drain =
+  # GUI modal in sub-track 4).
+  self._inject_preview_error:           Optional[ErrorInfo] = None
+  self._mcp_config_parse_error:         Optional[ErrorInfo] = None
+  self._save_project_error:             Optional[ErrorInfo] = None
   # --- Shared background pool + proactive warmup (startup_speedup_20260606) ---
   self._io_pool = make_io_pool()
   _install_sigint_exit_handler(self)
@@ -1518,15 +1526,27 @@ class AppController:
     Updates the preview content based on the selected file and injection mode.
     [C: src/gui_2.py:App._render_text_viewer_window, tests/test_skeleton_injection.py:test_update_inject_preview_full, tests/test_skeleton_injection.py:test_update_inject_preview_skeleton, tests/test_skeleton_injection.py:test_update_inject_preview_truncation]
   """
+  result = self._update_inject_preview_result()
+  if result.ok:
+   self._inject_preview = result.data
+   self._inject_preview_error = None
+  else:
+   err = result.errors[0]
+   self._inject_preview_error = err
+   self._inject_preview = f"Error reading file: {err.message}"
+
+ def _update_inject_preview_result(self) -> "Result[str]":
+  """Read the file for the inject preview. Returns Result[str] (Phase 6 Group 6.3).
+  On failure: OSError/IOError/UnicodeDecodeError -> ErrorInfo(original=e).
+  Caller (`_update_inject_preview`) stores the error on
+  `self._inject_preview_error` for sub-track 4 GUI."""
   if not self._inject_file_path:
-   self._inject_preview = ""
-   return
+   return Result(data="")
   target_path = self._inject_file_path
   if not os.path.isabs(target_path):
    target_path = os.path.join(self.active_project_root, target_path)
   if not os.path.exists(target_path):
-   self._inject_preview = ""
-   return
+   return Result(data="")
   try:
    with open(target_path, "r", encoding="utf-8") as f:
     content = f.read()
@@ -1538,10 +1558,14 @@ class AppController:
    lines = preview.splitlines()
    if len(lines) > 500:
     preview = "\n".join(lines[:500]) + "\n... (truncated)"
-   self._inject_preview = preview
+   return Result(data=preview)
   except (OSError, IOError, UnicodeDecodeError) as e:
-   logging.getLogger(__name__).debug("inject preview file read failed: %s", e, extra={"source": "app_controller._update_inject_preview"})
-   self._inject_preview = f"Error reading file: {e}"
+   return Result(data="", errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=str(e),
+    source="app_controller._update_inject_preview_result",
+    original=e,
+   )])
 
  @property
  def ai_status(self) -> str:
@@ -1684,11 +1708,30 @@ class AppController:
   return json.dumps(self.mcp_config.to_dict()) if self.mcp_config else "{}"
  @mcp_config_json.setter
  def mcp_config_json(self, value: str) -> None:
+  result = self._set_mcp_config_json_result(value)
+  if not result.ok:
+   err = result.errors[0]
+   self._mcp_config_parse_error = err
+   sys.stderr.write(f"mcp_config parse failed: {err.ui_message()}\n")
+   sys.stderr.flush()
+
+ def _set_mcp_config_json_result(self, value: str) -> "Result[None]":
+  """Parse and assign mcp_config from a JSON string. Returns Result[None] (Phase 6 Group 6.3).
+  On failure: json.JSONDecodeError/ValueError/TypeError/KeyError/AttributeError -> ErrorInfo(original=e).
+  Property setters cannot return values; this sibling helper carries the
+  structured result. Caller (`mcp_config_json` setter) stores the error
+  on `self._mcp_config_parse_error` for sub-track 4 GUI."""
   try:
    data = json.loads(value)
    self.mcp_config = models.MCPConfiguration.from_dict(data)
+   return OK
   except (json.JSONDecodeError, ValueError, TypeError, KeyError, AttributeError) as e:
-   logging.getLogger(__name__).debug("mcp config parse failed: %s", e, extra={"source": "app_controller.mcp_config_json"})
+   return Result(data=None, errors=[ErrorInfo(
+    kind=ErrorKind.INVALID_INPUT,
+    message=str(e),
+    source="app_controller._set_mcp_config_json_result",
+    original=e,
+   )])
 
  @property
  def ui_file_paths(self) -> list[str]:
@@ -3125,13 +3168,32 @@ class AppController:
   """
     [C: src/gui_2.py:App.delete_context_preset, src/gui_2.py:App.save_context_preset]
   """
-  if self.active_project_path:
-   try:
-    cleaned = project_manager.clean_nones(self.project)
-    project_manager.save_project(cleaned, self.active_project_path)
-   except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError) as e:
-    logging.getLogger(__name__).debug("save_project failed: %s", e, extra={"source": "app_controller._save_active_project"})
-    self.ai_status = f"save error: {e}"
+  result = self._save_active_project_result()
+  if not result.ok:
+   err = result.errors[0]
+   self._save_project_error = err
+   self.ai_status = f"save error: {err.message}"
+   sys.stderr.write(f"save_project failed: {err.ui_message()}\n")
+   sys.stderr.flush()
+
+ def _save_active_project_result(self) -> "Result[None]":
+  """Save the active project to its current path. Returns Result[None] (Phase 6 Group 6.3).
+  On failure: OSError/IOError/ValueError/TypeError/KeyError/AttributeError -> ErrorInfo(original=e).
+  Caller (`_save_active_project`) stores the error on
+  `self._save_project_error` for sub-track 4 GUI and updates self.ai_status."""
+  if not self.active_project_path:
+   return OK
+  try:
+   cleaned = project_manager.clean_nones(self.project)
+   project_manager.save_project(cleaned, self.active_project_path)
+   return OK
+  except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError) as e:
+   return Result(data=None, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=str(e),
+    source="app_controller._save_active_project_result",
+    original=e,
+   )])
 
  #endregion: Project