fix(plugin-harness): add no-op process_deferred_updates to test double (#391)

The safety harness's VisualTestDisplayManager (base of
BoundsCheckingDisplayManager) doesn't implement process_deferred_updates(),
which 5 first-party ledmatrix-plugins call unconditionally between
set_scrolling_state() and their scroll-position update: news, odds-ticker,
ledmatrix-leaderboard, stock-news, and ledmatrix-stocks. Any of them fails
the harness with AttributeError the moment it's touched (surfaced when
ledmatrix-plugins#177 had to add a local hasattr guard in ledmatrix-stocks
just to pass CI). Add the method as a no-op, mirroring the existing
"no-op for testing" pattern already used for set_scrolling_state, so these
plugins render under the harness without every touching PR needing its own
guard.

src/plugin_system/testing/visual_display_manager.py

@@ -454,6 +454,18 @@ class VisualTestDisplayManager:
         """Check if display is currently scrolling."""
         return self._scrolling_state['is_scrolling']
 
+    def process_deferred_updates(self):
+        """Process any deferred updates (no-op for testing).
+
+        Several ticker-style plugins (news, odds-ticker, leaderboard,
+        stock-news, stocks) call this unconditionally between
+        set_scrolling_state() and their scroll-position update, mirroring the
+        real display_manager's deferred-update queue. This double has no such
+        queue, so there is nothing to process — the no-op just lets those
+        plugins render under the harness instead of raising AttributeError.
+        """
+        pass
+
     # ------------------------------------------------------------------
     # Utility methods
     # ------------------------------------------------------------------