fix: Fr::from_u64 must use big-endian encoding to match C++ msgpack format

Fr::from_u64 was storing values in little-endian, but the C++ field::msgpack_unpack
expects big-endian (network byte order). This was masked before because the C++ side
silently reduced non-canonical values via to_montgomery_form_reduced(). After #22311
added strict non-canonical field validation (throwing for values >= modulus), values
like Fr::from_u64(123) produce 0x7B<<248 which exceeds the BN254 Fr modulus (0x30...)
and cause a C++ exception that propagates through the FFI boundary, aborting the Rust
test process.

Author: AztecBot

barretenberg/rust/barretenberg-rs/src/types.rs

@@ -7,10 +7,10 @@ use serde::{Deserialize, Serialize};
 pub struct Fr(pub [u8; 32]);
 
 impl Fr {
-    /// Create a new field element from a u64 value (little-endian encoding)
+    /// Create a new field element from a u64 value (big-endian encoding, matching C++ msgpack format)
     pub fn from_u64(value: u64) -> Self {
         let mut bytes = [0u8; 32];
-        bytes[0..8].copy_from_slice(&value.to_le_bytes());
+        bytes[24..32].copy_from_slice(&value.to_be_bytes());
         Fr(bytes)
     }