feat: add OnUpdate frame script for per-frame addon callbacks

Frames can now set an OnUpdate script that fires every frame with
the elapsed time as an argument. This enables addon timers, polling,
and animations.

  local f = CreateFrame("Frame")
  f:SetScript("OnUpdate", function(self, elapsed)
      -- called every frame with deltaTime
  end)

OnUpdate only fires for visible frames (frame:Hide() pauses it).
Tracked in __WoweeOnUpdateFrames table, dispatched via
LuaEngine::dispatchOnUpdate() called from the Application main loop.

src/core/application.cpp

@@ -1002,6 +1002,9 @@ void Application::update(float deltaTime) {
                     gameHandler->update(deltaTime);
                 }
             });
+            if (addonManager_ && addonsLoaded_) {
+                addonManager_->update(deltaTime);
+            }
             // Always unsheath on combat engage.
             inGameStep = "auto-unsheathe";
             updateCheckpoint = "in_game: auto-unsheathe";