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/change_notify.rs

@@ -0,0 +1,93 @@
+//! CHANGE_NOTIFY Request/Response (MS-SMB2 §2.2.35 / §2.2.36).
+//!
+//! V1 returns `STATUS_NOT_SUPPORTED`, but we still parse/encode the wire
+//! form so the dispatcher can recognize it.
+
+use binrw::{BinRead, BinWrite, binrw};
+use std::io::Cursor;
+
+use super::create::FileId;
+use crate::proto::error::ProtoResult;
+
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ChangeNotifyRequest {
+    pub structure_size: u16,
+    pub flags: u16,
+    pub output_buffer_length: u32,
+    pub file_id: FileId,
+    pub completion_filter: u32,
+    pub reserved: u32,
+}
+
+impl ChangeNotifyRequest {
+    /// Flag: SMB2_WATCH_TREE.
+    pub const FLAG_WATCH_TREE: u16 = 0x0001;
+
+    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(())
+    }
+}
+
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ChangeNotifyResponse {
+    pub structure_size: u16,
+    pub output_buffer_offset: u16,
+    pub output_buffer_length: u32,
+    #[br(count = output_buffer_length as usize)]
+    pub buffer: Vec<u8>,
+}
+
+impl ChangeNotifyResponse {
+    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 request_round_trips() {
+        let r = ChangeNotifyRequest {
+            structure_size: 32,
+            flags: ChangeNotifyRequest::FLAG_WATCH_TREE,
+            output_buffer_length: 0x1000,
+            file_id: FileId::new(1, 2),
+            completion_filter: 0xFF,
+            reserved: 0,
+        };
+        let mut buf = Vec::new();
+        r.write_to(&mut buf).unwrap();
+        assert_eq!(ChangeNotifyRequest::parse(&buf).unwrap(), r);
+    }
+
+    #[test]
+    fn response_round_trips() {
+        let r = ChangeNotifyResponse {
+            structure_size: 9,
+            output_buffer_offset: 0x48,
+            output_buffer_length: 0,
+            buffer: vec![],
+        };
+        let mut buf = Vec::new();
+        r.write_to(&mut buf).unwrap();
+        assert_eq!(ChangeNotifyResponse::parse(&buf).unwrap(), r);
+    }
+}