SMB performance optimization: pread/pwrite, tokio::sync::Mutex, direct response, fast-path

- VfsFile trait: add read_at()/write_at() with seek+read default impl
- LocalFs: override with real pread/pwrite (FileExt::read_at/write_at) — 1 syscall vs 2
- smb_server_backend: use read_at/write_at + tokio::sync::Mutex (non-blocking async)
- read handler: build response directly, avoid Bytes→Vec<u8> copy + intermediate struct
- oplock break: fast-path skip when ≤1 open entry (single-user scenario)

vendor/smb-server/src/handlers/read.rs

@@ -91,16 +91,15 @@ pub async fn handle(
     if bytes.is_empty() && req.length > 0 {
         return HandlerResponse::err(ntstatus::STATUS_END_OF_FILE);
     }
-    let resp = ReadResponse {
-        structure_size: 17,
-        data_offset: ReadResponse::STANDARD_DATA_OFFSET,
-        reserved: 0,
-        data_length: bytes.len() as u32,
-        data_remaining: 0,
-        flags: 0,
-        data: bytes.to_vec(),
-    };
-    let mut buf = Vec::new();
-    resp.write_to(&mut buf).expect("encode");
+    // Build response directly to avoid Bytes→Vec<u8> copy and intermediate struct
+    let data_len = bytes.len() as u32;
+    let mut buf = Vec::with_capacity(16 + bytes.len());
+    buf.extend_from_slice(&17u16.to_le_bytes());      // structure_size
+    buf.push(ReadResponse::STANDARD_DATA_OFFSET);      // data_offset
+    buf.push(0);                                       // reserved
+    buf.extend_from_slice(&data_len.to_le_bytes());    // data_length
+    buf.extend_from_slice(&0u32.to_le_bytes());        // data_remaining
+    buf.extend_from_slice(&0u32.to_le_bytes());        // flags
+    buf.extend_from_slice(&bytes);                     // data
     HandlerResponse::ok(buf)
 }