Add S3 VFS backend: VfsBackend impl for S3-compatible storage

- S3Vfs with all 15 VfsBackend methods via rusty-s3 + ureq
- S3VfsFile for buffered writes + ranged reads
- AWS Signature V4 pre-signed URLs (rusty-s3)
- ListObjectsV2 for directory listing (prefix + delimiter)
- Path-style URL mapping (/path to bucket/key)

markbase-core/Cargo.toml

@@ -59,6 +59,9 @@ aes = "0.8"
 ctr = "0.9"
 cipher = "0.4"
 nix = { version = "0.29", features = ["poll", "fs"] }  # Phase 14: OpenSSH风格的poll()和非阻塞I/O（fs feature包含fcntl）
+rusty-s3 = "0.10"     # S3 API 签名（AWS Signature V4）
+ureq = "2.12"          # 輕量同步 HTTP 客戶端
+url = "2"              # URL 解析（rusty-s3 依賴）
 
 [features]
 default = []                          # 默认不启用可选格式