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/src/proto/messages/echo.rs

@@ -0,0 +1,83 @@
+//! ECHO Request/Response (MS-SMB2 §2.2.28).
+
+use binrw::{BinRead, BinWrite, binrw};
+use std::io::Cursor;
+
+use crate::proto::error::ProtoResult;
+
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct EchoRequest {
+    pub structure_size: u16,
+    pub reserved: u16,
+}
+
+impl Default for EchoRequest {
+    fn default() -> Self {
+        Self {
+            structure_size: 4,
+            reserved: 0,
+        }
+    }
+}
+
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct EchoResponse {
+    pub structure_size: u16,
+    pub reserved: u16,
+}
+
+impl Default for EchoResponse {
+    fn default() -> Self {
+        Self {
+            structure_size: 4,
+            reserved: 0,
+        }
+    }
+}
+
+impl EchoRequest {
+    pub fn parse(buf: &[u8]) -> ProtoResult<Self> {
+        Ok(Self::read(&mut Cursor::new(buf))?)
+    }
+    pub fn write_to(&self, out: &mut Vec<u8>) -> ProtoResult<()> {
+        let mut c = Cursor::new(Vec::new());
+        BinWrite::write(self, &mut c)?;
+        out.extend_from_slice(&c.into_inner());
+        Ok(())
+    }
+}
+
+impl EchoResponse {
+    pub fn parse(buf: &[u8]) -> ProtoResult<Self> {
+        Ok(Self::read(&mut Cursor::new(buf))?)
+    }
+    pub fn write_to(&self, out: &mut Vec<u8>) -> ProtoResult<()> {
+        let mut c = Cursor::new(Vec::new());
+        BinWrite::write(self, &mut c)?;
+        out.extend_from_slice(&c.into_inner());
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trips() {
+        let req = EchoRequest::default();
+        let mut buf = Vec::new();
+        req.write_to(&mut buf).unwrap();
+        assert_eq!(buf.len(), 4);
+        assert_eq!(EchoRequest::parse(&buf).unwrap(), req);
+
+        let resp = EchoResponse::default();
+        let mut buf = Vec::new();
+        resp.write_to(&mut buf).unwrap();
+        assert_eq!(EchoResponse::parse(&buf).unwrap(), resp);
+    }
+}