feat(lee_core): add length-prefixed frame helpers for the zkVM boundary

lee/state_machine/core/src/frame.rs

@@ -0,0 +1,58 @@
+//! Length-prefixed byte framing for the zkVM I/O boundary.
+//!
+//! A frame is a 4-byte little-endian length prefix followed by the payload bytes. The same
+//! [`to_frame`] layout is used by the guest journal commit, the circuit's `env::verify`
+//! reconstruction, and the host input write, so all sides agree on the exact byte sequence and the
+//! recursion journal digests match. [`from_frame`] recovers the payload on the host, ignoring any
+//! trailing transport bytes beyond the prefixed length.
+
+/// Frames `payload` as a 4-byte little-endian length prefix followed by the payload bytes.
+#[must_use]
+pub fn to_frame(payload: &[u8]) -> Vec<u8> {
+    let len = u32::try_from(payload.len()).expect("frame payload length must fit in u32");
+    let mut framed = len.to_le_bytes().to_vec();
+    framed.extend_from_slice(payload);
+    framed
+}
+
+/// Returns the payload slice of a frame produced by [`to_frame`], ignoring any bytes past the
+/// prefixed length (e.g. transport word-alignment padding).
+#[must_use]
+pub fn from_frame(bytes: &[u8]) -> &[u8] {
+    let (len_bytes, payload) = bytes.split_at(4);
+    let len = usize::try_from(u32::from_le_bytes(
+        len_bytes
+            .try_into()
+            .expect("frame must have a 4-byte length prefix"),
+    ))
+    .expect("frame length must fit in usize");
+    &payload[..len]
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{from_frame, to_frame};
+
+    #[test]
+    fn frame_round_trip() {
+        let payload: &[u8] = b"hello borsh boundary";
+        let framed = to_frame(payload);
+        assert_eq!(from_frame(&framed), payload);
+    }
+
+    #[test]
+    fn frame_tolerates_trailing_padding() {
+        let payload: &[u8] = &[1, 2, 3, 4, 5, 6, 7];
+        let mut framed = to_frame(payload);
+        // Simulate transport padding the frame up to a word boundary.
+        framed.extend_from_slice(&[0, 0, 0]);
+        assert_eq!(from_frame(&framed), payload);
+    }
+
+    #[test]
+    fn empty_payload_round_trips() {
+        let framed = to_frame(&[]);
+        assert_eq!(framed, vec![0, 0, 0, 0]);
+        assert!(from_frame(&framed).is_empty());
+    }
+}

lee/state_machine/core/src/lib.rs

@@ -15,6 +15,7 @@ pub use encryption::{
     EncryptedAccountData, EncryptionScheme, EphemeralPublicKey, EphemeralSecretKey,
     ML_KEM_768_CIPHERTEXT_LEN, SharedSecretKey, ViewTag,
 };
+pub use frame::{from_frame, to_frame};
 pub use nullifier::{
     AuthorizationSecretKey, Identifier, Nullifier, NullifierPublicKey, NullifierSecretKey,
 };
@@ -25,6 +26,7 @@ mod circuit_io;
 mod commitment;
 mod encoding;
 pub mod encryption;
+mod frame;
 mod nullifier;
 pub mod program;