Add per-weapon VXL turret switching for MultiWeapon units

[NukeAtty]

Adds support for swapping VXL turret and barrel models per weapon index for units using MultiWeapon=yes together with TurretCount>0.

Summary

• Units with MultiWeapon=yes + TurretCount>0 now swap their VXL turret and barrel based on the currently selected weapon.
• Extra models are loaded by naming convention (<ImageFile>turN.vxl / <ImageFile>barlN.vxl); no artmd.ini changes required.
• The number of effective turrets is the smaller of WeaponCount and TurretCount; weapons beyond that range fall back to the default turret.
• Elite weapons follow the same per-index rule.
• Turret and barrel shadows are drawn as usual when UseTurretShadow=yes.

Implementation

• UnitTypeExt loads the extra VXL/HVA pairs (LoadMultiWeaponTurrets) and frees them on destruction.
• UnitClass_AI sets CurrentTurretNumber from SelectWeapon.
• Draw (0x73BA12) and shadow (0x73C47A) hooks route the turret/barrel voxel per weapon index, bypassing the voxel cache for custom turrets.
• Defensive skip in the charge turret hook (0x6FA540).

Notes

• Does not apply to units with IsChargeTurret=yes, IsGattling=yes or Gunner=yes.

[TaranDahl]

The unit will lose its target when you issue an order with a mouse click, so I suspect this will lead to very strange in‑game behavior.
What specific use case are you trying to achieve with it?

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[NukeAtty]

The unit will lose its target when you issue an order with a mouse click, so I suspect this will lead to very strange in‑game behavior. What specific use case are you trying to achieve with it?

e.g. Use AA turret when firing AA weapon.

[TaranDahl]

I think a timer could be set so that the turret remains active for a period of time after the unit loses its target.
Looking forward to other maintainers' opinions about the whole design.

[DeathFishAtEase]

Changelog position is incorrect.

[DeathFishAtEase]

If it does not require any new statements and is used in conjunction with MultiWeapon, then it is best to merge it into the paragraph where the MultiWeapon feature resides.