Creature: fix SetBaseAC/GetBaseAC to use m_nACPolymorph when polymorphed

[silent-rock]

Problem

When a creature is polymorphed, the engine uses m_nACPolymorph (in CNWSCreatureStats) for AC calculation instead of m_nACNaturalBase. The existing SetBaseAC and GetBaseAC functions only communicated with m_nACNaturalBase, making them ineffective on polymorphed creatures.
This was confirmed on our server after investigating why a feat-based natural AC bonus (granted on polymorph) had no effect. The value written by SetBaseAC was correctly stored in m_nACNaturalBase, but the engine ignored it entirely during polymorph - using m_nACPolymorph instead. Similarly, GetBaseAC was returning the value from m_nACNaturalBase even while polymorphed, which did not reflect the actual AC being used by the engine.

Related discussion: https://nwn.zulipchat.com/#narrow/channel/574443-nwnx/topic/NWNX_Creature_SetBaseAC.20has.20no.20effect.20on.20polymorphed/with/583022185

Fix

• Added GetBaseACPolymorph() and SetBaseACPolymorph() to directly access m_nACPolymorph, which is the variable used by the engine for AC calculation while polymorphed.
• Added a note to GetBaseAC()/SetBaseAC() documentation clarifying that these functions do not affect AC while polymorphed, and pointing to the new functions.