Retail compatibility adjustments (Merchant API, PvP taint, Secret guards)

[SpeedysGames]

This PR proposes minimal compatibility adjustments for the current Retail build (tested against the latest S&L Alpha).

Changes:

• Merchant: Retail API migration (GetMerchantItemInfo -> C_MerchantFrame.GetItemInfo), robust icon mapping, and extended-cost fallback for currency/token vendors.
• Nameplates: guard invalid threat status input and avoid comparing secret GUID values.
• Friends datatext: guard secret system messages before string operations.
• PvP module: register events via ElvUI dispatcher to avoid AceEvent protected RegisterEvent path (ADDON_ACTION_FORBIDDEN).
• Professions deconstruct: defensive overlay glow handling to avoid nil calls.

Tested locally in Retail with BugGrabber enabled (Altoholic active). No Lua errors observed after applying.

Related: #134