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/MacroLengthWarn.lua

@@ -133,12 +133,6 @@ function Extension.OnMacroFrameLoad()
     end
 end
 
-function Extension.OnAddonLoaded()
-    if arg1 == "Blizzard_MacroUI" then
-        Extension.OnMacroFrameLoad()
-    end
-end
-
 function Extension.OnLoad()
     -- Schedule messages to show after UI is ready
     local function ShowMessages()
@@ -178,16 +172,11 @@ function Extension.OnLoad()
         end
     end
 
-    -- Listen for macro UI loading
-    Extension.RegisterEvent("ADDON_LOADED", "OnAddonLoaded")
+    -- Hook the macro UI once available (fires immediately if already loaded).
+    EventUtil.ContinueOnAddOnLoaded("Blizzard_MacroUI", Extension.OnMacroFrameLoad)
 
-    -- Also try to hook MacroFrame_SaveMacro if it already exists
-    if MacroFrame_SaveMacro then
-        Extension.OnMacroFrameLoad()
-    end
-
-    -- Register PLAYER_LOGIN to show status messages
-    Extension.RegisterEvent("PLAYER_LOGIN", "OnPlayerLogin")
+    -- Status messages on login (currently disabled inside OnPlayerLogin).
+    EventUtil.ContinueOnPlayerLogin(Extension.OnPlayerLogin)
 
     -- Store the message function for later
     Extension.ShowMessages = ShowMessages