refactor: commit to 8 byte obfuscation keys

Since 31 byte xor-keys are not used in the codebase, using the common size (8 bytes) makes the benchmarks more realistic.

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

src/util/obfuscation.h

@@ -0,0 +1,16 @@
+// Copyright (c) 2025-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_OBFUSCATION_H
+#define BITCOIN_UTIL_OBFUSCATION_H
+
+#include <cstdint>
+
+class Obfuscation
+{
+public:
+    static constexpr size_t KEY_SIZE{sizeof(uint64_t)};
+};
+
+#endif // BITCOIN_UTIL_OBFUSCATION_H