nameplates: give the name colour a single owner

cache.namecolor was written by two writers holding unrelated quantities:
the computed health-bar colour in OnDataChanged, which class-colours
friendly player names when friendclassnamec is on, and Blizzard's own
name FontString in the OnUpdate sync. Sharing one key let either suppress
the other, so the option silently lost - names came out white with
namefightcolor off, or the plain reaction colour with it on.

It also forced a full data pass every throttled tick, because the sync
writer sets update = true whenever it writes and the two kept flipping
the key. That defeats the half-second tick gate entirely.

Compounded by nameplate.cache surviving pool reuse: OnShow re-runs the
data pass but never clears the table, which is built once at plate
creation, so a recycled plate could inherit the previous unit's name
colour.

Ownership is now explicit through cache.ownname, each writer keeps its
own key, and both are invalidated when ownership flips.

(cherry picked from commit 0a4b3a7d6ae523cf70584adbb694a8740024b7cd)
(cherry picked from commit 0df9bb9fa907349182fb03217cab6aa1777aa7d9)

modules/nameplates.lua

@@ -1260,9 +1260,25 @@ nameplates:RegisterEvent("PLAYER_GUILD_UPDATE")
       plate.cache.r, plate.cache.g, plate.cache.b = r, g, b
     end
 
-    if r + g + b ~= plate.cache.namecolor and unittype == "FRIENDLY_PLAYER" and C.nameplates["friendclassnamec"] == "1" and class and PFUI_CLASS_COLORS[class] then
+    -- Friendly player names take this colour when friendclassnamec is on.
+    -- Ownership is recorded so the OnUpdate sync below stands down rather than
+    -- racing us: both writers used to share cache.namecolor despite storing
+    -- unrelated quantities (this colour vs Blizzard's name FontString), so
+    -- either could suppress the other -- and since nameplate.cache survives
+    -- pool reuse, a recycled plate could keep the previous unit's name colour.
+    local ownname = unittype == "FRIENDLY_PLAYER" and C.nameplates["friendclassnamec"] == "1"
+      and class and PFUI_CLASS_COLORS[class] and true or nil
+
+    if plate.cache.ownname ~= ownname then
+      plate.cache.ownname = ownname
+      -- ownership flipped: whichever writer is now in charge must re-assert
+      plate.cache.namecolor = nil
+      plate.cache.ownnamecolor = nil
+    end
+
+    if ownname and r + g + b ~= plate.cache.ownnamecolor then
+      plate.cache.ownnamecolor = r + g + b
       plate.name:SetTextColor(r, g, b, a)
-      plate.cache.namecolor = r + g + b
     end
 
     if target and C.nameplates.cpdisplay == "1" then
@@ -1530,24 +1546,28 @@ nameplates:RegisterEvent("PLAYER_GUILD_UPDATE")
       update = true
     end
 
-    -- trigger update when name color changed (includes combat state check)
-    local r, g, b = original.name:GetTextColor()
-    local inCombatWithPlayer = cfg.namefightcolor and UnitAffectingCombat(nameplate.unit) and UnitAffectingCombat("player")
-    
-    if r + g + b ~= nameplate.cache.namecolor or (cfg.namefightcolor and nameplate.cache.inCombat ~= inCombatWithPlayer) then
-      nameplate.cache.namecolor = r + g + b
-      nameplate.cache.inCombat = inCombatWithPlayer
+    -- trigger update when name color changed (includes combat state check).
+    -- Skipped once OnDataChanged owns the name colour (class-coloured friendly
+    -- players), so the two writers cannot overwrite each other.
+    if not nameplate.cache.ownname then
+      local r, g, b = original.name:GetTextColor()
+      local inCombatWithPlayer = cfg.namefightcolor and UnitAffectingCombat(nameplate.unit) and UnitAffectingCombat("player")
 
-      if cfg.namefightcolor then
-        if (r > .9 and g < .2 and b < .2) or inCombatWithPlayer then
-          nameplate.name:SetTextColor(1,0.4,0.2,1)
+      if r + g + b ~= nameplate.cache.namecolor or (cfg.namefightcolor and nameplate.cache.inCombat ~= inCombatWithPlayer) then
+        nameplate.cache.namecolor = r + g + b
+        nameplate.cache.inCombat = inCombatWithPlayer
+
+        if cfg.namefightcolor then
+          if (r > .9 and g < .2 and b < .2) or inCombatWithPlayer then
+            nameplate.name:SetTextColor(1,0.4,0.2,1)
+          else
+            nameplate.name:SetTextColor(r,g,b,1)
+          end
         else
-          nameplate.name:SetTextColor(r,g,b,1)
+          nameplate.name:SetTextColor(1,1,1,1)
         end
-      else
-        nameplate.name:SetTextColor(1,1,1,1)
+        update = true
       end
-      update = true
     end
 
     -- trigger update when level color changed. Skipped while the level came