test(gui_2): add 2 Phase 3 invariant tests + Phase 3 checkpoint

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

Phase 3 covered (8 INTERNAL_BROAD_CATCH sites migrated to Result[T]):
- L731 _load_fonts main font [53412af1]
- L742 _load_fonts mono font [61cf4055]
- L1123 _gui_func render [0f102612]
- L1171 _show_menus do_generate [bcbd4644]
- L1197 _show_menus hwnd [f51abe07]
- L1222 _show_menus is_max [44e28889]
- L1284 _handle_history_logic [500108ea]
- L4848 render_warmup_status_indicator [0dacbfce]

Each site has a _result helper that returns Result[bool] with ErrorInfo
on failure; the legacy wrapper routes errors to the appropriate data
plane attribute (_last_request_errors, _startup_timeline_errors,
or _worker_errors).

Audit: V=30 (down from 38), COMPLIANT=20 (up from 12). Tests: 22/22 pass.
Phase 3 invariant tests added:
- test_phase_3_invariant_batch_a_count_dropped: verifies 17 INTERNAL_BROAD_CATCH
  remain (was 25; dropped 8).
- test_phase_3_invariant_all_8_migration_sites_have_tests: verifies all 8
  sites have both success and failure tests.

Phase 4 begins: INTERNAL_BROAD_CATCH Batch B (3 modal/dialog sites).

tests/test_gui_2_result.py

@@ -481,6 +481,58 @@ def test_phase_3_l1284_handle_history_logic_result_success():
  The simplest success path is when _last_ui_snapshot is None (first
  snapshot, early return) or when nothing changed (no push needed).
  """
+
+
+def test_phase_3_invariant_batch_a_count_dropped():
+ """
+ Phase 3 invariant: the audit's INTERNAL_BROAD_CATCH count for src/gui_2.py
+ has dropped from 25 to 17 (a drop of 8 sites).
+
+ The 8 migrated sites are: L731 (main font), L742 (mono font), L1123
+ (_gui_func), L1171 (_show_menus do_generate), L1197 (_show_menus hwnd),
+ L1222 (_show_menus is_max), L1284 (_handle_history_logic), L4848
+ (render_warmup_status_indicator).
+ """
+ result = subprocess.run(
+  ["uv", "run", "python", "scripts/audit_exception_handling.py", "--src", "src", "--json"],
+  capture_output=True,
+  text=True,
+ )
+ assert result.returncode == 0, (
+  f"audit_exception_handling.py exited {result.returncode}; stderr:\n"
+  f"{result.stderr[:2000]}"
+ )
+ data = json.loads(result.stdout)
+ gui2 = [f for f in data.get("files", []) if "gui_2" in f.get("filename", "")][0]
+ broad_catches = [f for f in gui2.get("findings", []) if f.get("category") == "INTERNAL_BROAD_CATCH"]
+ assert len(broad_catches) == 17, (
+  f"Phase 3 invariant: expected 17 INTERNAL_BROAD_CATCH sites in src/gui_2.py "
+  f"(down from 25); found {len(broad_catches)}. The 8 Batch A sites must be "
+  f"migrated to Result[T] helpers. Lines: {[f.get('line') for f in broad_catches]}"
+ )
+
+
+def test_phase_3_invariant_all_8_migration_sites_have_tests():
+ """
+ Phase 3 invariant: each of the 8 Batch A sites has both success and
+ failure tests in this test file.
+ """
+ import re
+ text = Path(__file__).read_text(encoding="utf-8")
+ # Expected: each line number in {731, 742, 1123, 1171, 1197, 1222, 1284, 4848}
+ # should have both _success and _failure tests
+ expected_lines = [731, 742, 1123, 1171, 1197, 1222, 1284, 4848]
+ for line in expected_lines:
+  success_pattern = f"test_phase_3_l{line}_.*_success"
+  failure_pattern = f"test_phase_3_l{line}_.*_failure"
+  assert re.search(success_pattern, text), (
+   f"Phase 3 invariant: missing success test for L{line}. "
+   f"Expected a test matching '{success_pattern}'."
+  )
+  assert re.search(failure_pattern, text), (
+   f"Phase 3 invariant: missing failure test for L{line}. "
+   f"Expected a test matching '{failure_pattern}'."
+  )
  from src import gui_2
  from unittest.mock import MagicMock
  app = MagicMock()