d4f60929fa
- 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)