P1: AsyncS3Vfs native async implementation using reqwest

- Replace spawn_blocking + ureq with native async reqwest
- AsyncS3Vfs uses reqwest::Client for HTTP operations
- rusty-s3 for presigned URL generation + XML parsing
- AsyncS3File with async read/write/seek/flush
- reqwest dependency added under async-vfs feature

Tests: 297 passed (293 + 4 new s3_auth tests)

markbase-core/Cargo.toml

@@ -69,6 +69,7 @@ chacha20poly1305 = "0.10"  # Phase 5: ChaCha20-Poly1305 AEAD（备用）
 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
+reqwest = { version = "0.12", optional = true }  # Async HTTP client for AsyncS3Vfs
 rayon = "1.10"          # Phase 4: 并行加密
 url = "2"              # URL 解析（rusty-s3 依賴）
 
@@ -85,7 +86,7 @@ tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 default = []                          # 默认不启用可选格式
 optional-formats = ["unrar", "xz2", "sevenz-rust"]  # 争议格式可选启用
 smb-server = ["dep:smb-server"]       # SMB server feature flag
-async-vfs = []                        # Async VfsBackend trait (Phase 1 - design only)
+async-vfs = ["dep:reqwest"]           # Async VfsBackend trait + native async S3
 
 [dev-dependencies]
 # tempfile moved to dependencies (needed for archive extraction)