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.

pfUI.lua

@@ -102,6 +102,10 @@ pfUI.version = {}
 pfUI.hooks = {}
 pfUI.env = {}
 
+pfUI.events = Mixin({}, CallbackRegistryMixin)
+pfUI.events:OnLoad()
+pfUI.events:SetUndefinedEventsAllowed(true)
+
 -- check if macro addons are loaded (disables macrotweak/macroscan)
 function pfUI:MacroAddonsLoaded()
   return IsAddOnLoaded("Supermacro") or IsAddOnLoaded("SuperCleveRoidMacros") or IsAddOnLoaded("UltimaMacros")