test: replace can't-fail tests with real assertions

test_font_manager.py was 5 of 6 tests shaped as 'try: call(); assert True /
except: assert True' — running in CI while unable to fail on any
regression. Rewritten against the real FontManager API and the bundled
assets/fonts: returned font types, cache-hit identity, distinct entries per
size, default-font fallback for unknown families and corrupt files
(recorded in failed_loads), BDF native-size reading, text measurement, and
cache lifecycle.

test_display_manager.py's test_draw_text ended in 'assert True'; it now
renders onto a known-black canvas and asserts pixels were actually lit —
which required un-breaking the fixture's freetype MagicMock so draw_text's
isinstance check doesn't silently swallow the draw.

test_display_controller.py carried a permanently-skipped test whose skip
reason already declared it redundant; deleted.

Both display test files now set EMULATOR=true before importing
display_manager (the same convention as test_display_dirty_tracking.py) so
they collect standalone instead of depending on which test module imports
display_manager first.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

test/test_display_controller.py

@@ -15,13 +15,6 @@ class TestDisplayControllerInitialization:
         assert test_display_controller.plugin_manager is not None
         assert test_display_controller.available_modes == []
 
-    @pytest.mark.skip(reason="No assertions; init logic is covered by test_init_success and fixture setup")
-    def test_plugin_discovery_and_loading(self, test_display_controller):
-        """Test plugin discovery and loading during initialization."""
-        pm = test_display_controller.plugin_manager
-        pm.discover_plugins.return_value = ["plugin1", "plugin2"]
-        pm.get_plugin.return_value = MagicMock()
-
 
 class TestDisplayControllerModeRotation:
     """Test display mode rotation logic."""