fuzz: fix ChaCha20 encrypt_single_block to preserve data

The fuzzing ChaCha20 implementation's encrypt_single_block was not
copying src to dest, causing encrypted data to be lost (dest remained
zeros). This broke payment flows where metadata is encrypted into
payment_secret - the receiver would decrypt zeros and detect the wrong
payment method (LdkPaymentHash instead of UserPaymentHash), causing
payments to fail with "mismatching preimage".

Fix by making encrypt_single_block copy src to dest (identity
encryption), matching the behavior of the process() method.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: joostjager

lightning/src/crypto/chacha20.rs

@@ -321,6 +321,7 @@ mod fuzzy_chacha {
 		) {
 			debug_assert_eq!(dest.len(), src.len());
 			debug_assert!(dest.len() <= 32);
+			dest.copy_from_slice(src);
 		}
 
 		pub fn encrypt_single_block_in_place(