TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 10: refactor(gui_2): migrate L612 _post_init callback to Result[T] (Phase 10 site 3)

Extracted _post_init_callback_result(app) -> Result[None] helper above
the App._post_init method.
ANTI-SLIMING: full Result[T] propagation (NO pass-after-logging). 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 _post_init method preserves its signature and calls the helper,
draining errors to self._startup_timeline_errors.

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

Audit: L612 reclassified from INTERNAL_SILENT_SWALLOW (10 sites remaining,
was 11). New helper L612 is INTERNAL_COMPLIANT.

tests/test_gui_2_result.py

@@ -1815,4 +1815,46 @@ def test_phase_10_l264_resolve_font_path_result_is_relative_to_raises():
   assert err.source == "gui_2._resolve_font_path_result"
 
 
+def test_phase_10_l612_post_init_callback_result_success():
+ """
+ L612 _post_init_callback_result returns Result(data=None) on success.
+
+ The helper extracts the warmup-complete callback registration from
+ App._post_init into a Result-returning helper. On success, it registers
+ the lambda callback via self.controller.on_warmup_complete() and returns
+ Result(data=None).
+ """
+ from unittest.mock import MagicMock
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app.controller = MagicMock()
+ result = gui2_mod._post_init_callback_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is None
+ app.controller.on_warmup_complete.assert_called_once()
+
+
+def test_phase_10_l612_post_init_callback_result_failure():
+ """
+ L612 _post_init_callback_result returns Result(data=None, errors=[ErrorInfo]) on failure.
+
+ When self.controller.on_warmup_complete() raises (e.g., controller not
+ ready or invalid callback), the helper converts to ErrorInfo and returns
+ Result(data=None, errors=[ErrorInfo]). The legacy _post_init wrapper
+ drains to self._startup_timeline_errors.
+ """
+ from unittest.mock import MagicMock
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app.controller = MagicMock()
+ app.controller.on_warmup_complete.side_effect = RuntimeError("controller not ready")
+ result = gui2_mod._post_init_callback_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.data is None
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._post_init_callback_result"
+ assert "controller not ready" in err.message
+
+