fix: corrupt low-order bytes in batch verifier test to avoid non-canonical field encoding (#22333)

## Summary

Fixes CI failure in `merge-train/barretenberg` caused by the interaction
of #22233 (Fr::from_u64 big-endian encoding) and #22311 (strict
non-canonical field validation).

The `corruptProofFields()` test helper was flipping high-order bytes
(indices 0,1) of field elements, creating values exceeding the BN254 Fr
modulus. The strict validation now rejects these at deserialization time
with `non-canonical encoding (value >= modulus)` before they reach the
verifier.

**Fix**: Flip low-order bytes (indices 30,31) instead — still corrupts
the proof but keeps field values canonical.

## Test plan
- [x] `batch_verifier.test.ts` — all 4 tests pass locally (previously 1
failing)

ClaudeBox log: https://claudebox.work/s/e5146dc92fea29dd?run=1

Author: AztecBot

yarn-project/ivc-integration/src/batch_verifier_test_helpers.ts

@@ -5,8 +5,8 @@ import type { IVCProofVerificationResult } from '@aztec/stdlib/interfaces/server
 export function corruptProofFields(fields: Uint8Array[]): Uint8Array[] {
   const corrupted = fields.map(f => Uint8Array.from(f));
   corrupted[2] = Uint8Array.from(corrupted[2]);
-  corrupted[2][0] ^= 0xff;
-  corrupted[2][1] ^= 0xff;
+  corrupted[2][30] ^= 0xff;
+  corrupted[2][31] ^= 0xff;
   return corrupted;
 }