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/addons/addon_manager.cpp

@@ -89,6 +89,10 @@ void AddonManager::fireEvent(const std::string& event, const std::vector<std::st
     luaEngine_.fireEvent(event, args);
 }
 
+void AddonManager::update(float deltaTime) {
+    luaEngine_.dispatchOnUpdate(deltaTime);
+}
+
 void AddonManager::shutdown() {
     addons_.clear();
     luaEngine_.shutdown();