perf: raise server MAX_READ_SIZE to SFTP standard 255 KiB

The bssh-server hard-capped every SFTP `READ` reply at 64 KiB
(`MAX_READ_SIZE = 65536`) regardless of what the client requested.
`bssh-russh-sftp` and OpenSSH's `sftp-server` both use the SFTP standard
`MAX_READ_LENGTH = 261120` (255 KiB) for request sizing, so a client
asking for a 256 KiB chunk only ever got 64 KiB back, forcing it to
issue four extra requests for the same byte stream.

Bump `MAX_READ_SIZE` to `261120` so server replies match the standard
chunk size used by the rest of the stack.  Combined with client-side
pipelining (#196), this directly cuts the per-MiB request count on
downloads from 16 → 4.

Memory exposure stays bounded: handles are still capped at
`MAX_HANDLES = 1000` per session and each in-flight read still uses
a single per-request buffer of this size (max ~255 KiB × in-flight
requests).

Author: Yaminyam

src/server/sftp.rs

@@ -164,8 +164,15 @@ struct DirEntryInfo {
 /// Maximum number of open handles per session to prevent resource exhaustion.
 const MAX_HANDLES: usize = 1000;
 
-/// Maximum read buffer size (64KB) to prevent memory exhaustion.
-const MAX_READ_SIZE: u32 = 65536;
+/// Maximum read buffer size per request.  Matches the SFTP standard
+/// `MAX_READ_LENGTH` (255 KiB) used by `bssh-russh-sftp` and OpenSSH
+/// `sftp-server`.  The previous 64 KiB cap silently truncated client `READ`
+/// requests for 256 KiB chunks down to 64 KiB, multiplying request count 4×
+/// for the same byte stream and dragging down sustained download throughput.
+/// Memory exposure remains bounded because handles are capped at
+/// [`MAX_HANDLES`] per session and each in-flight read uses a single
+/// per-request buffer of this size.
+const MAX_READ_SIZE: u32 = 261120;
 
 /// Normalize a path's `..` and `.` components without touching the filesystem.
 ///