fix: surface speed effect removal, type correctness, and code cleanup

[plenarius]

Summary

• Fix Expeditious Retreat being stripped by road transitions: Effect removal now uses RemoveEffectByTag with ID-based removal instead of deferred AIMasterEvent::RemoveEffect with raw CGameEffect* pointers, which could match and remove unrelated MovementSpeedIncrease effects
• Fix undefined engine behavior for slowing surfaces: Uses MovementSpeedDecrease with std::abs(effectChange) for negative speed modifiers (mud, water, snow, sand) instead of MovementSpeedIncrease with negative values
• Fix unbounded map growth: Uses find() on m_SurfaceMaterialSpeeds instead of operator[] which silently inserted default entries for every unknown surface material
• Fix RemoveEffectByTag calling RemoveEffectById(0) 128 times: vector(128) pre-filled 128 zeros before real IDs; changed to reserve(128)
• Fix server crash on malformed rune tags: Extracted ParseRuneTag helper with bounds checking; callers guard against sentinel return
• Fix OOB vector access in NextColor/PreviousColor: Added bounds checks before accessing v[index] at vector boundaries
• Allow weapon color 0: SetItemAppearance now accepts val >= 0 and NextColor wrap-around searches from 0
• Code cleanup: Extracted GetRaceVFXOffset helper (3x dedup), removed dead m_GetItemInSlotHook member, removed duplicate #include, fixed %d → %zu for size_t

Fixes nwnxee#374

Test plan

• Cast Expeditious Retreat, walk onto a road tile and back off — speed bonus should persist
• Walk through mud/water/snow/sand — movement should slow correctly
• Walk between road and non-road tiles rapidly — no effect stacking
• Use rune system (apply, combine, describe) — functionality unchanged
• Cycle weapon colors forward/backward at boundaries — no crash
• Set weapon color to 0 — should work