Replaced session.read(path) (buffers entire file in memory, OOM risk
for large files) with session.open(path) + SftpReader — a streaming
Read bridge that wraps an open SFTP file handle and reads in bounded
chunks via AsyncReadExt::read + block_on.
SftpReader uses Mutex<File> for thread safety and tokio::runtime::Handle
to bridge async→sync. Each Read::read() call issues a single SFTP read
request for a bounded chunk — large files are never buffered in memory.
Updated module doc to document the streaming read surface.
1433 tests pass (default), 1442 with --features sftp, zero warnings.
vasilito
1c8cb18d8a