Event-drive nameplate target state; drop per-tick poll and targetPlateGuid

frameState.hasTarget/targetGuid were polled via UnitExists("target") every
central-loop tick, while targetPlateGuid held the same guid but was already
event-driven -- redundant. Set frameState target fields in PLAYER_TARGET_CHANGED
(seeded on PLAYER_ENTERING_WORLD for a target held across reload), remove the
poll, and collapse targetPlateGuid into frameState.targetGuid (the castbar
frame now reads that). The central loop's only remaining per-tick poll is
GetTime().

modules/nameplates.lua

@@ -73,7 +73,6 @@ pfUI:RegisterModule("nameplates", function ()
   -- guid -> nameplate, maintained on NAME_PLATE_UNIT_ADDED/_REMOVED so a cast
   -- event can find its plate in O(1) and only cache casts we actually show.
   local plateByGuid = {}
-  local targetPlateGuid = nil
 
   -- One-shot C_Spell poll. Only used to seed a plate that spawns while its
   -- unit is already mid-cast (its SPELL_START_OTHER fired before the plate
@@ -499,7 +498,7 @@ nameplates:RegisterEvent("SPELL_FAILED_OTHER")
         CacheConfig()
         this:SetGameVariables()
         RebuildRaidGuidCache()
-        targetPlateGuid = UnitExists("target") and UnitGUID("target") or nil
+        frameState.hasTarget, frameState.targetGuid = UnitExists("target"), UnitGUID("target")
       end
       
       -- Handle friendly zone nameplate disable feature
@@ -662,7 +661,7 @@ nameplates:RegisterEvent("SPELL_FAILED_OTHER")
       end
 
     elseif event == "PLAYER_TARGET_CHANGED" then
-      targetPlateGuid = UnitExists("target") and UnitGUID("target") or nil
+      frameState.hasTarget, frameState.targetGuid = UnitExists("target"), UnitGUID('target')
       -- Flag the target's plate for update
       local plate = C_NamePlate.GetNamePlateForUnit("target")
       if plate and plate.nameplate then
@@ -691,7 +690,6 @@ nameplates:RegisterEvent("SPELL_FAILED_OTHER")
 
     -- PERF: Cache GetTime() once per frame
     frameState.now = now
-    frameState.hasTarget, frameState.targetGuid = UnitExists("target")
 
     if this.eventcache then
       this.eventcache = nil
@@ -1671,8 +1669,8 @@ nameplates:RegisterEvent("SPELL_FAILED_OTHER")
   -- loop's nameplates_castbar gate.)
   nameplates.castbarFrame = CreateFrame("Frame", nil, UIParent)
   nameplates.castbarFrame:SetScript("OnUpdate", function()
-    if not cfg.showcastbar or not targetPlateGuid then return end
-    local nameplate = plateByGuid[targetPlateGuid]
+    if not cfg.showcastbar or not frameState.targetGuid then return end
+    local nameplate = plateByGuid[frameState.targetGuid]
     if not nameplate then return end
     nameplates.UpdateCastbar(nameplate, GetTime())
   end)