bench: make ObfuscationBench more representative

A previous PR already solved the tiny byte-array-xors during serialization, so it makes sense to keep focusing on the performance of bigger continuous chunks.

This also renames the file from `xor` to `obfuscation` to enable scripted diff name unification later.

> C++ compiler .......................... GNU 14.2.0

|             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|                0.84 |    1,184,138,235.64 |    0.0% |            9.01 |            3.03 |  2.971 |           1.00 |    0.1% |      5.50 | `ObfuscationBench`

> C++ compiler .......................... Clang 20.1.7

|             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|                0.89 |    1,124,087,330.23 |    0.1% |            6.52 |            3.20 |  2.041 |           0.50 |    0.2% |      5.50 | `ObfuscationBench`

Author: l0rinc

src/bench/CMakeLists.txt

@@ -35,6 +35,7 @@ add_executable(bench_bitcoin
   mempool_eviction.cpp
   mempool_stress.cpp
   merkle_root.cpp
+  obfuscation.cpp
   parse_hex.cpp
   peer_eviction.cpp
   poly1305.cpp
@@ -51,7 +52,6 @@ add_executable(bench_bitcoin
   txgraph.cpp
   util_time.cpp
   verify_script.cpp
-  xor.cpp
 )
 
 include(TargetDataSources)

src/bench/xor.cpp renamed to src/bench/obfuscation.cpp

@@ -4,22 +4,23 @@
 
 #include <bench/bench.h>
 #include <random.h>
-#include <span.h>
 #include <streams.h>
 #include <util/obfuscation.h>
 
 #include <cstddef>
 #include <vector>
 
-static void Xor(benchmark::Bench& bench)
+static void ObfuscationBench(benchmark::Bench& bench)
 {
     FastRandomContext frc{/*fDeterministic=*/true};
     auto data{frc.randbytes<std::byte>(1024)};
-    auto key{frc.randbytes<std::byte>(Obfuscation::KEY_SIZE)};
+    const auto key{frc.randbytes<Obfuscation::KEY_SIZE>()};
 
+    size_t offset{0};
     bench.batch(data.size()).unit("byte").run([&] {
-        util::Xor(data, key);
+        util::Xor(data, key, offset++); // mutated differently each time
+        ankerl::nanobench::doNotOptimizeAway(data);
     });
 }
 
-BENCHMARK(Xor, benchmark::PriorityLevel::HIGH);
+BENCHMARK(ObfuscationBench, benchmark::PriorityLevel::HIGH);