SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

vendor/smb2/Cargo.toml

@@ -0,0 +1,81 @@
+[package]
+name = "smb2"
+version = "0.11.3"
+edition = "2021"
+rust-version = "1.85"
+license = "MIT OR Apache-2.0"
+description = "Pure-Rust SMB2/3 client library with pipelined I/O"
+repository = "https://github.com/vdavid/smb2"
+keywords = ["smb", "smb2", "smb3", "cifs", "network"]
+categories = ["network-programming", "filesystem"]
+readme = "README.md"
+documentation = "https://docs.rs/smb2"
+exclude = [
+    ".github/",
+    "AGENTS.md",
+    "docs/",
+    "justfile",
+    "deny.toml",
+    "clippy.toml",
+    "rustfmt.toml",
+    "related-repos/",
+]
+
+[package.metadata.docs.rs]
+all-features = true
+
+[dependencies]
+# Logging facade -- application picks the backend (env_logger, tracing, etc.)
+log = "0.4"
+
+# Async runtime agnostic
+async-trait = "0.1"
+
+# `FuturesUnordered` for pipelined concurrent `execute` calls.
+futures-util = { version = "0.3", default-features = false, features = ["std", "async-await"] }
+
+# Error handling
+thiserror = "2"
+
+# Enum conversion derives
+num_enum = "0.7"
+
+# Async runtime -- transport layer needs net, io-util, time, sync
+tokio = { version = "1", features = ["net", "io-util", "time", "sync", "rt"] }
+
+# Crypto -- signing, encryption, key derivation
+hmac = "0.13"
+sha2 = "0.11"
+aes = "0.9"
+aes-gcm = "=0.11.0-rc.4"
+ccm = "=0.6.0-rc.3"
+cmac = "=0.8.0-rc.5"
+digest = "0.11"
+
+# NTLM authentication (MS-NLMP)
+md-5 = "0.11"
+md4 = "0.11"
+
+# Kerberos key derivation (AES string-to-key)
+pbkdf2 = "0.13"
+sha1 = "0.11"
+
+# Cryptographically secure random
+getrandom = "0.4"
+
+# Compression
+lz4_flex = "0.13"
+
+# Optional: `Serialize` derives on diagnostics types. Off by default.
+serde = { version = "1", optional = true, features = ["derive"] }
+
+[features]
+testing = []  # Enables smb2::testing module for Docker-based test servers
+fuzzing = []  # Exposes parser entry points for `fuzz/` targets; not for applications
+serde = ["dep:serde"]  # `Serialize` impls on `Diagnostics` types and the protocol enums they embed.
+
+[dev-dependencies]
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time", "net", "io-util"] }
+proptest = "1"
+env_logger = "0.11"
+serde_json = "1"  # JSON round-trip tests for the `serde` feature