feat: let a frame reader hand back what it read past a frame

The end that reads the first frame of an attach stream to learn whom the
connection is for then forwards the rest untouched, and what arrived behind that
frame in the same read belongs to the stream.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/nesprotocol/src/attach.rs

@@ -135,6 +135,15 @@ impl FrameReader {
         self.buf.extend_from_slice(bytes);
     }
 
+    /// What has arrived and is not yet a frame, handing the reader up.
+    ///
+    /// For the end that reads the first frame to learn whom a connection is for
+    /// and then forwards the rest untouched: whatever came in behind that frame
+    /// in the same read belongs to the stream, and is not to be lost.
+    pub fn into_remaining(self) -> Vec<u8> {
+        self.buf
+    }
+
     /// The next whole frame, if one has arrived.
     ///
     /// `Ok(None)` is "not yet". After an error the reader is poisoned in the
@@ -256,6 +265,19 @@ mod tests {
         }
     }
 
+    /// The first frame is read to learn who is calling; whatever came in behind it
+    /// in the same read is the stream's, and is not dropped.
+    #[test]
+    fn what_arrived_behind_a_frame_is_handed_on() {
+        let mut reader = FrameReader::new();
+        let mut wire = Frame::Hello("a".into()).encode().unwrap();
+        wire.extend(Frame::Data(b"prompt$ ".to_vec()).encode().unwrap());
+        reader.push(&wire);
+        assert_eq!(reader.next_frame().unwrap(), Some(Frame::Hello("a".into())));
+        let rest = reader.into_remaining();
+        assert_eq!(rest, Frame::Data(b"prompt$ ".to_vec()).encode().unwrap());
+    }
+
     #[test]
     fn two_frames_in_one_read_come_out_as_two() {
         let mut reader = FrameReader::new();