refactor(gui_2): migrate L742 _load_fonts mono font to Result[T] (Phase 3)

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

Adds _load_fonts_mono_result(app, font_size, config) -> Result[bool] helper
that wraps the thirdparty hello_imgui.FontLoadingParams + hello_imgui.load_font
try/except from App._load_fonts. App._load_fonts becomes a thin wrapper that
drains errors to _startup_timeline_errors (startup-time error plane).

Audit: BROAD_CATCH count 24 -> 23, COMPLIANT count 13 -> 14. Tests: 2/2 pass.

tests/test_gui_2_result.py

@@ -249,4 +249,50 @@ def test_phase_3_l731_load_fonts_main_result_failure():
  assert result.errors, "Expected at least one error on failure"
  err = result.errors[0]
  assert err.source == "gui_2._load_fonts_main_result"
- assert "font load failed" in err.message
+ assert "font load failed" in err.message
+
+
+def test_phase_3_l742_load_fonts_mono_result_success():
+ """
+ L742 _load_fonts_mono_result returns Result.ok=True on success.
+
+ The helper wraps the mono font loading try/except in App._load_fonts.
+ On success, it returns Result(data=True) with no errors and sets
+ app.mono_font to the loaded font.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ mock_mono_font = MagicMock(name="mock_mono_font")
+ mock_config    = MagicMock(name="mock_font_config")
+ with patch.object(gui_2, "hello_imgui") as mock_hi, \
+   patch("src.startup_profiler.startup_profiler") as mock_sp:
+  mock_hi.FontLoadingParams.return_value = "mock_params"
+  mock_hi.load_font.return_value         = mock_mono_font
+  result = gui_2._load_fonts_mono_result(app, 16.0, mock_config)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+ assert app.mono_font is mock_mono_font
+
+
+def test_phase_3_l742_load_fonts_mono_result_failure():
+ """
+ L742 _load_fonts_mono_result returns Result.ok=False with ErrorInfo on failure.
+
+ When the underlying third-party hello_imgui.load_font call raises, the
+ helper converts the exception to ErrorInfo and returns Result(data=False).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ mock_config = MagicMock(name="mock_font_config")
+ with patch.object(gui_2, "hello_imgui") as mock_hi, \
+   patch("src.startup_profiler.startup_profiler") as mock_sp:
+  mock_hi.FontLoadingParams.return_value              = "mock_params"
+  mock_hi.load_font.side_effect                       = RuntimeError("mono font missing")
+  result = gui_2._load_fonts_mono_result(app, 16.0, mock_config)
+ 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._load_fonts_mono_result"
+ assert "mono font missing" in err.message