Harden strsplit against third-party global clobbering

BigWigs (SpellRequests) redefines the global string:split to return a table,
and another addon clobbers the strsplit global too. pfUI.api.strsplit and the
bare strsplit callers inherited the broken versions depending on load order,
producing 'attempt to compare number with nil' from GetStringColor.

- Make pfUI.api.strsplit fully self-contained (no delegation to global
  strsplit / string.split), so no override can reach it.
- Optimize the hot path: single-char delimiters use plain-text find (no
  pattern compilation, no per-call char-class string); localize string.find
  and string.sub.
- Route the per-frame castbar/nameplate color splits through GetStringColor,
  which caches, instead of re-splitting a constant string every update.

modules/castbar.lua

@@ -90,7 +90,7 @@ pfUI:RegisterModule("castbar", function ()
     cb:SetAlpha(1)
     cb.fadeout = nil
 
-    cb.bar:SetStatusBarColor(strsplit(",", C.appearance.castbar[isChannel and "channelcolor" or "castbarcolor"]))
+    cb.bar:SetStatusBarColor(GetStringColor(C.appearance.castbar[isChannel and "channelcolor" or "castbarcolor"]))
 
     local rank = ""
     if spellID then
@@ -103,8 +103,7 @@ pfUI:RegisterModule("castbar", function ()
     if tex and cb.showicon then
       local size = cb:GetHeight()
       cb.icon:Show()
-      cb.icon:SetHeight(size)
-      cb.icon:SetWidth(size)
+      cb.icon:SetSize(size, size)
       cb.icon.texture:SetTexture(tex)
       cb.bar:SetPoint("TOPLEFT", cb.icon, "TOPRIGHT", cb.spacing, 0)
     else
@@ -169,8 +168,7 @@ pfUI:RegisterModule("castbar", function ()
     -- icon
     cb.icon = CreateFrame("Frame", nil, cb)
     cb.icon:SetPoint("TOPLEFT", 0, 0)
-    cb.icon:SetHeight(16)
-    cb.icon:SetWidth(16)
+    cb.icon:SetSize(16, 16)
 
     cb.icon.texture = cb.icon:CreateTexture(nil, "OVERLAY")
     cb.icon.texture:SetAllPoints()

modules/nameplates.lua

@@ -1570,7 +1570,7 @@ nameplates:RegisterEvent("UNIT_FLAGS")
       -- Relative 0..duration range to avoid float precision loss with large
       -- absolute timestamps.
       nameplate.castbar:SetMinMaxValues(0, duration)
-      nameplate.castbar:SetStatusBarColor(strsplit(",", C.appearance.castbar[(isChannel and "channelcolor" or "castbarcolor")]))
+      nameplate.castbar:SetStatusBarColor(GetStringColor(C.appearance.castbar[(isChannel and "channelcolor" or "castbarcolor")]))
       if castInfo.icon then
         nameplate.castbar.icon.tex:SetTexture(castInfo.icon)
         nameplate.castbar.icon.tex:SetTexCoord(.1,.9,.1,.9)