fix: Fr::from_u64 big-endian encoding to match C++ msgpack format (#22233)

## Summary

Fixes bb-rs test failure introduced by #22311 (graceful failures in
verifier code paths).

`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 (`0x30644e72...`) and cause a C++ exception
that propagates through the FFI boundary, aborting the Rust test process
with "Rust cannot catch foreign exceptions".

## Fix

One-line change: store the u64 value in bytes 24..32 as big-endian
instead of bytes 0..8 as little-endian.

## Test plan

- [x] Reproduced failure locally — `test_pedersen_commit_deterministic`
crashes with SIGABRT
- [x] Applied fix — all 70 bb-rs tests pass (3 perf tests ignored)
- [ ] `./bootstrap.sh ci` running

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)
     }