character: durability % per inventory slot

Adds a Fizzle-style durability label to each character pane slot.
Renders at the bottom of the slot icon as "N%" colored by the existing
DURABILITY_THRESHOLD_COLORS table (red < orange < yellow < invasion <
green), or hidden when the item has no durability stat (necks, rings,
trinkets) or the slot is empty.

api/api.lua: new pfUI.api.CreateFontString(f, key, layer, size, flags,
font) helper following the CreateBackdrop pattern — idempotent
get-or-create that attaches the FontString to the parent frame as the
named field. Defaults to pfUI.font_default at C.global.font_size with
OUTLINE on the OVERLAY layer.

skins/blizzard/character.lua: scoreText migrated from the inline
"if not frame.scoreText then ..." block to the new helper as a working
example; new durabilityText created the same way at BOTTOM/size 10.
Durability render added to RefreshCharacterSlot — runs on every
PaperDollItemSlotButton_Update so equip/unequip/repair/damage all
refresh automatically.

api/config.lua + modules/gui.lua: new C.character.inventory.durability
toggle (default "1"). GUI lives under a new top-level "Character" tab
→ "Inventory" sub-tab, leaving room for future "Reputation" / "Skills"
sub-tabs at the same level.

api/config.lua

@@ -913,6 +913,8 @@ function pfUI:LoadConfig()
   pfUI:UpdateConfig("gm",         nil,           "server",           "elysium")
 
   pfUI:UpdateConfig("questlog",   nil,           "showQuestLevels",  "0")
+
+  pfUI:UpdateConfig("character",  "inventory",   "durability",       "1")
   pfUI:UpdateConfig("thirdparty", nil,           "chatbg",           "1")
   pfUI:UpdateConfig("thirdparty", nil,           "showmeter",        "0")
   pfUI:UpdateConfig("thirdparty", "dpsmate",     "skin",             "0")