add pfUI.events callback registry, replace addoncompat OnUpdate poll

Introduce a central pfUI.events registry (ClassicAPI's
CallbackRegistryMixin, undefined events allowed) initialized in pfUI.lua
before any module body runs, so publishers/subscribers don't depend on
module load order.

firstrun sets `pfUI.firstrun.completed` and fires `firstrun:complete` at
the point NextStep detects no pending steps. PLAYER_ENTERING_WORLD re-
enters this path on every zone, so the flag is a one-shot guard.

addoncompat drops its 0.1s OnUpdate poll and either RunQueues immediately
(returning user, all steps already done) or subscribes to the event.

modules/firstrun.lua

@@ -46,6 +46,11 @@ pfUI:RegisterModule("firstrun", function ()
         return
       end
     end
+
+    if not self.completed then
+      self.completed = true
+      pfUI.events:TriggerEvent("firstrun:complete")
+    end
   end
 
   -- main function to create wizard windows