feat: 무기 특성(WeaponTrait) 시스템 및 보조손 장착 구현

- WeaponTrait enum 추가 (Melee/TwoHanded/Defense/Magic/Ranged)

- SkillData에 allowedWeaponTraits 필드 및 MatchesWeaponTrait() 검증 메서드 추가

- WeaponEquipment에 보조손(오프핸드) 슬롯 지원 (장착/해제/스탯 보너스/네트워크 동기화)

- EquippedWeaponTraits 프로퍼티로 메인+보조 무기 특성 합산 제공

- PlayerSkillInput 4곳(OnSkillInput/RPC/CanUseSkill/DebugExecute)에 무기 trait 제약 검증 추가

- SkillSlotUI에 무기 불호환 시 회색 표시(weaponIncompatibleColor) 지원

- 기존 검 에셋에 weaponTrait=Melee 설정

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