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.

include/addons/lua_engine.hpp

@@ -32,6 +32,9 @@ public:
     // Try to dispatch a slash command via SlashCmdList. Returns true if handled.
     bool dispatchSlashCommand(const std::string& command, const std::string& args);
 
+    // Call OnUpdate scripts on all frames that have one.
+    void dispatchOnUpdate(float elapsed);
+
     lua_State* getState() { return L_; }
     bool isInitialized() const { return L_ != nullptr; }