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/addoncompat.lua

@@ -134,19 +134,9 @@ pfUI:RegisterModule("addoncompat", function ()
   end
 
   -- run the addonconflict queue when firstrun is ready
-  local delay = CreateFrame("Frame")
-  delay:SetScript("OnUpdate", function()
-    -- throttle to to one query per .1 second
-    if ( this.tick or 1) > GetTime() then return else this.tick = GetTime() + .1 end
-
-    -- make sure the firstrun dialog has finished
-    if pfUI.firstrun and pfUI.firstrun.steps then
-      for _, step in pairs(pfUI.firstrun.steps) do
-        if not pfUI_init[step.name] then return end
-      end
-    end
-
+  if pfUI.firstrun and pfUI.firstrun.completed then
     RunQueue()
-    this:SetScript("OnUpdate", nil)
-  end)
+  else
+    pfUI.events:RegisterCallback("firstrun:complete", RunQueue, "addoncompat")
+  end
 end)