TankAggroBar

Defensively handle WoW secret (tainted/restricted) GUID strings when tracking threat: use pcall + tostring to obtain a safe GUID, fall back to the unit token when necessary, and guard accesses to the threatCache with pcall to avoid secret string table key errors. Also avoid treating a nil/unsafe GUID as a change when comparing cached guids. Additionally, bump addon version in the .toc from 1.5 to 1.6.