perf(Core): Optimize damage/heal hot paths and gate Mythic AI

[Nyeriah]

• Cache SpellBuff/MeleeBuff OnlyBosses config bools at load time; avoids per-hit string lookups via sConfigMgr->GetOption.
• Replace mutating NerfInfo[mapId][phase] reads with find()/cached refs to stop std::map::operator[] from inserting empty entries on hot damage/heal paths. Also fixes a silent-insert in the debug branch of ModifySpellDamageTaken when matchingPhase == -1.
• Switch NerfInfo (outer), SpellNerfOverrides, MythicmodeInstanceData, and Expansion from std::map to std::unordered_map. NerfInfo's inner map stays std::map because GetLowestMatchingPhase relies on ordered iteration.
• Add ModZoneDifficulty.MythicmodeAI.Enable (default 0) so OnUnitEnterCombat can early-out without doing IsInstanceMythic / MythicmodeAI lookups when the scripted AI is unused.
• Guard attacker null in the OnlyBosses checks (caster can be null for periodic ticks; matches the existing debug-branch guard).