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/smb-server/Cargo.toml

@@ -0,0 +1,57 @@
+[package]
+name = "smb-server"
+version = "0.4.1"
+edition = "2024"
+rust-version = "1.95"
+license = "MIT"
+repository = "https://github.com/paltaio/rust-smb-server"
+description = "SMB2/3 file-sharing server library with pluggable storage backends."
+
+[dependencies]
+tokio = { version = "1.40", features = ["full"] }
+bytes = "1.7"
+async-trait = "0.1"
+tracing = "0.1"
+thiserror = "1"
+uuid = { version = "1.10", features = ["v4"] }
+binrw = "0.15"
+getrandom = "0.4"
+cap-std = { version = "3", optional = true }
+hmac = "0.12"
+sha2 = "0.10"
+md-5 = "0.10"
+md4 = "0.10"
+aes = "0.8"
+cmac = "0.7"
+rc4 = "0.2"
+
+[features]
+default = ["localfs"]
+localfs = ["dep:cap-std"]
+
+[dev-dependencies]
+tracing-subscriber = "0.3"
+tempfile = "3"
+hex = "0.4"
+
+[[test]]
+name = "integration_localfs"
+path = "tests/integration_localfs.rs"
+required-features = ["localfs"]
+
+[[test]]
+name = "integration_localfs_write"
+path = "tests/integration_localfs_write.rs"
+required-features = ["localfs"]
+
+[[test]]
+name = "integration_negotiate"
+path = "tests/integration_negotiate.rs"
+required-features = ["localfs"]
+
+[profile.release]
+opt-level = 3
+lto = true
+codegen-units = 1
+panic = "abort"
+strip = true