phase 2 removing tbc

Strip dead pfUI.client > 11200 and pfUI.expansion == "tbc" branches now
that vanilla is the only supported client.

- pfUI.lua: hardcode expansion = "vanilla"; collapse force_region matrix
- compat/vanilla.lua, libs/{libcast,libdebuff,libtotem,libunitscan,
  focus}: drop always-false early-returns
- api/unitframes.lua: delete TargetBuffOnUpdate poller + tbc visibility
  and secure-template branches; click-cast keeps only vanilla path
- api/ui-widgets.lua: unconditional EffectiveScale divide
- env/tables.lua: drop unused focus/focustarget pfValidUnits entries
- modules/actionbar.lua: simplify EnablePaging, drop ButtonSwitch/
  petvisibility, unwrap prowl block, single keybind-remap path
- modules/{castbar,chat,cooldown,loot,questitem,totems}: unwrap
  always-true conditionals
- modules/nameplates.lua: cfg-only fake-cooldown gate, unwrap overlap/
  clickthrough + mouselook blocks
- modules/friends.lua + skins/blizzard/friends.lua: maxtab = 4
- skins/blizzard/{help,game_menu,questlog}: drop tbc-only branches

modules/loot.lua

@@ -714,14 +714,10 @@ pfUI:RegisterModule("loot", "vanilla:tbc", function ()
       if event == "LOOT_BIND_CONFIRM" then
         local slot = arg1
         QueueFunction(function()
-          if pfUI.client <= 12000 then
-            LootSlot(slot)
-          elseif pfUI.client <= 20400 then
-            ConfirmLootSlot(slot)
-          end
+          LootSlot(slot)
           StaticPopup_Hide("LOOT_BIND")
         end)
-      elseif event == "LOOT_OPENED" and pfUI.client <= 11200 then
+      elseif event == "LOOT_OPENED" then
         for i=1,GetNumLootItems() do
           LootSlot(i)
         end