refactor(gui_2): migrate L1284 _handle_history_logic to Result[T] (Phase 3)

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 3.

Adds _handle_history_logic_result(app) -> Result[bool] helper that wraps
the snapshot debounce try/except from App._handle_history_logic. The
_is_applying_snapshot pre-condition guard stays in the legacy wrapper
(not error handling; the original early return has no try/except).

App._handle_history_logic becomes a thin wrapper that drains errors to
_last_request_errors. The drain failure mode is structurally safe
(hasattr check + append) so no outer try/except is required (per the
L1123 wrapper decision; avoiding new INTERNAL_SILENT_SWALLOW violations).

Audit: BROAD_CATCH count 19 -> 18, COMPLIANT count 18 -> 19. Tests: 2/2 pass.

tests/test_gui_2_result.py

@@ -463,4 +463,49 @@ def test_phase_3_l1222_show_menus_is_max_result_failure():
  assert result.errors, "Expected at least one error on failure"
  err = result.errors[0]
  assert err.source == "gui_2._show_menus_is_max_result"
- assert result.data is False
+ assert result.data is False
+
+
+def test_phase_3_l1284_handle_history_logic_result_success():
+ """
+ L1284 _handle_history_logic_result returns Result.ok=True on success.
+
+ The helper wraps the snapshot try/except in App._handle_history_logic.
+ The simplest success path is when _last_ui_snapshot is None (first
+ snapshot, early return) or when nothing changed (no push needed).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ app._is_applying_snapshot = False
+ app._last_ui_snapshot     = None
+ mock_snapshot             = MagicMock(name="mock_snapshot")
+ mock_snapshot.disc_entries = []
+ mock_snapshot.files        = []
+ mock_snapshot.context_files = []
+ mock_snapshot.screenshots  = []
+ app._take_snapshot.return_value = mock_snapshot
+ result = gui_2._handle_history_logic_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+
+
+def test_phase_3_l1284_handle_history_logic_result_failure():
+ """
+ L1284 _handle_history_logic_result returns Result.ok=False with ErrorInfo on failure.
+
+ When _take_snapshot raises (or any other code in the try body), the
+ helper returns Result(data=False, errors=[ErrorInfo]).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ app._is_applying_snapshot = False
+ app._last_ui_snapshot     = MagicMock()
+ app._take_snapshot.side_effect = ValueError("snapshot failed")
+ result = gui_2._handle_history_logic_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._handle_history_logic_result"
+ assert "snapshot failed" in err.message