fix(party): set oUF-guessUnit fallback to prevent nil index in oUF

oUF ouf.lua:270 calls objectUnit:match() which crashes when both
the unit parameter and oUF-guessUnit attribute are nil. This happens
in solo test mode when the header creates frames with no real party
member. Set a 'party' fallback in the initialConfigFunction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: danvernon

EllesmereUIUnitFrames/EllesmereUIPartyFrames.lua

@@ -404,6 +404,10 @@ local function SpawnPartyHeader()
         "oUF-initialConfigFunction", ([[
             self:SetWidth(%d)
             self:SetHeight(%d)
+            -- Ensure oUF-guessUnit is never nil (oUF crashes at ouf.lua:270 otherwise)
+            if(not self:GetAttribute('oUF-guessUnit')) then
+                self:SetAttribute('oUF-guessUnit', 'party')
+            end
         ]]):format(settings.frameWidth or 160, (settings.healthHeight or 36) + ((settings.powerPosition ~= "none") and (settings.powerHeight or 4) or 0)),
     }