feat: 채널링 빔 스킬 시스템 구현 및 PolygonParticleFX VFX 에셋 추가

- SkillData: 채널링 필드 추가 (지속시간, 틱 간격, 틱/종료 효과, VFX 프리팹, 마운트 경로, 크기 배율)
- SkillController: 채널링 상태 관리 (Start/Update/End), VFX 생성/파괴, 틱 효과 주기 발동, 버튼 해제로 중단
- SkillEffect: Beam(원통) 범위 판정 추가 (OverlapCapsule), 디버그 시각화
- PlayerSkillInput: 스킬 취소(canceled) 이벤트 구독 → 채널링 중단 통지
- SkillLoadoutEntry: 채널링 틱/종료 효과 수집 메서드 추가
- 스킬 데이터/이펙트/애니메이션/VFX 에셋 추가 (채널링 스킬용)
- PolygonParticleFX VFX 에셋 패키지 추가 (Materials, Models, Prefabs, Textures, Scenes)

Assets/External/VFX/PolygonParticleFX/Models/SM_LightRayRound 1.fbx.meta

@@ -0,0 +1,95 @@
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+    100002: Light_Shaft_Manip
+    100004: Null
+    100006: SM_LightRay_Round
+    100008: //RootNode
+    400000: Light_Shaft_Base
+    400002: Light_Shaft_Manip
+    400004: Null
+    400006: SM_LightRay_Round
+    400008: //RootNode
+    4300000: SM_LightRay_Round
+    9500000: //RootNode
+    13700000: SM_LightRay_Round
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+    legStretch: 0.05
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