Use ClassicAPI EventUtil for addon-load/login extension wiring

Replace hand-rolled ADDON_LOADED/PLAYER_LOGIN handlers with
EventUtil.ContinueOnAddOnLoaded / ContinueOnPlayerLogin, which fire
immediately if the event already happened -- removing the "we loaded before
the target addon and missed its ADDON_LOADED" workarounds.

- pfUI compat: ContinueOnAddOnLoaded("pfUI") + ContinueOnPlayerLogin; drops
  the missed-event fallback (login path still re-runs SetupCompatibility).
- MacroErrorUI / MacroLengthWarn: ContinueOnAddOnLoaded("Blizzard_MacroUI"),
  folding their manual "already loaded" checks.
- 9 Mouseover extensions (ag_UnitFrames, CT_RaidAssist, CT_UnitFrames,
  DiscordUnitFrames, Grid, NotGrid, Cursive, sRaidFrames, PerfectRaid):
  ContinueOnAddOnLoaded("<AddonName>", OnLoad). Since immediate-fire passes no
  event args, the old `arg1 == "X"` checks are replaced by the addon-name gate
  (global guards kept where present); also removes the buggy
  UnregisterEvent("ADDON_LOADED", "Onload") no-ops that never fired.

Addon names match file names; drops support for renamed folders (e.g. -master).

Extensions/Mouseover/DiscordUnitFrames.lua

@@ -6,7 +6,6 @@ local _G = _G or getfenv(0)
 local CleveRoids = _G.CleveRoids or {}
 
 local Extension = CleveRoids.RegisterExtension("DiscordUnitFrames")
-Extension.RegisterEvent("ADDON_LOADED", "OnLoad")
 
 function Extension.OnEnterFrame()
     CleveRoids.SetMouseoverFrom("duf", this.unit)
@@ -27,10 +26,6 @@ function Extension.OnLeaveElement()
 end
 
 function Extension.OnLoad()
-    if arg1 ~= "DiscordUnitFrames" then
-        return
-    end
-
     CleveRoids.ClearHooks()
     Extension.Hook("DUF_UnitFrame_OnEnter", "OnEnterFrame")
     Extension.Hook("DUF_UnitFrame_OnLeave", "OnLeaveFrame")
@@ -39,4 +34,4 @@ function Extension.OnLoad()
     Extension.Hook("DUF_Element_OnLeave", "OnLeaveElement")
 end
 
-_G["CleveRoids"] = CleveRoids
+EventUtil.ContinueOnAddOnLoaded("DiscordUnitFrames", Extension.OnLoad)