Route slash registration through pfUI.api.RegisterSlashCommand

The RegisterSlashCommand helper in api/api.lua was effectively unused
(only macrotweak called it); every other command hand-rolled the
SLASH_*/SlashCmdList pair. Convert the existing manual registrations to
the helper with force=true, preserving the current always-bind behavior
while centralizing the pattern behind one code path (and its _G. and
conflict-check handling).

Left as-is: pfUI.lua's /rl, /pfui, /gm (registered before api.lua
defines the helper) and the vendored libs' debug commands.

modules/unitxp.lua

@@ -352,8 +352,7 @@ pfUI:RegisterModule("unitxp", function ()
   end
 
   -- Debug command to test UnitXP indicators
-  _G.SLASH_PFUNITXP1 = "/pfunitxp"
-  SlashCmdList["PFUNITXP"] = function()
+  pfUI.api.RegisterSlashCommand("PFUNITXP", { "/pfunitxp" }, function()
     local chat = DEFAULT_CHAT_FRAME
     chat:AddMessage("|cff33ffccpfUI|r: UnitXP Indicator Debug")
 
@@ -387,5 +386,5 @@ pfUI:RegisterModule("unitxp", function ()
     else
       chat:AddMessage("  Target frame: |cffff0000NOT found|r")
     end
-  end
+  end, true)
 end)