feat: 전투 공간 개편 및 카메라·캐릭터 품질 개선

[전투 공간]
- 단순 Plane 바닥 → Arena 계층 구조로 교체 (Floor, 벽 4개, Objects)
- PolygonDarkFortress 외부 에셋 임포트 (전투 공간 디자인 적용)
- 바닥 아래 검은 평면(FloorBase) 추가로 카메라 저각도 시 허공 노출 방지
- NavMesh 리베이크

[카메라]
- PlayerCamera에 SphereCast 기반 지형 충돌 감지 추가
- 카메라가 바닥 아래를 비출 때 최소 높이 보장

[캐릭터]
- 플레이어 CharacterController skinWidth 정상화 (0.0001 → 0.03)
- 보스 NavMeshAgent baseOffset 조정으로 발 파묻힘 수정

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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