TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 10: refactor(gui_2): migrate L1647 render_main_interface focus_response to Result[T] (Phase 10 site 8)

Extracted _focus_response_window_result() -> Result[None] helper above
the call site in render_main_interface.
ANTI-SLIMING: full Result[T] propagation (NO bare-except+pass). The
helper returns Result(data=None) on success or Result(data=None,
errors=[ErrorInfo]) on exception (logging NOT a drain per the user's
principle 2026-06-17).

The legacy render_main_interface code preserves its behavior, calls
the helper, drains errors to app._last_request_errors.

Tests: 2 new tests verify both paths (success and RuntimeError).

Audit: L1647 reclassified from INTERNAL_SILENT_SWALLOW (6 sites remaining,
was 7). New helper L1647 is INTERNAL_COMPLIANT.

src/gui_2.py

@@ -1644,10 +1644,32 @@ def render_main_interface(app: App) -> None:
  if app.controller._process_pending_tool_calls(): app._tool_log_dirty = True
  if app._pending_focus_response:
   app._pending_focus_response = False
-  try:
-   imgui.set_window_focus("Response") # type: ignore[call-arg]
-  except:
-   pass
+  focus_result = _focus_response_window_result()
+  if not focus_result.ok:
+   if not hasattr(app, '_last_request_errors'): app._last_request_errors = []
+   app._last_request_errors.append(("render_main_interface.focus_response", focus_result.errors[0]))
+
+def _focus_response_window_result() -> Result[None]:
+ """Drain-aware variant of render_main_interface imgui.set_window_focus try block (L1647 INTERNAL_SILENT_SWALLOW).
+
+ Extracts the thirdparty imgui.set_window_focus("Response") try/except from
+ render_main_interface into a Result-returning helper. On exception (native
+ bundle error, IM_ASSERT), converts to ErrorInfo (logging NOT a drain per
+ the user's principle 2026-06-17). The caller drains to
+ app._last_request_errors.
+
+ [C: src/gui_2.py:render_main_interface (L1647 legacy wrapper)]
+ """
+ try:
+  imgui.set_window_focus("Response") # type: ignore[call-arg]
+  return Result(data=None)
+ except Exception as e:
+  return Result(data=None, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"imgui.set_window_focus('Response') failed: {e}",
+   source="gui_2._focus_response_window_result",
+   original=e,
+  )])
  #endregion: Process GUI task queue
  
  render_track_proposal_modal(app)