optimization: migrate SipHashUint256 to PresaltedSipHasher

Replaces standalone `SipHashUint256` with an `operator()` overload in `PresaltedSipHasher`.
Updates all hasher classes (`SaltedUint256Hasher`, `SaltedTxidHasher`, `SaltedWtxidHasher`) to use `PresaltedSipHasher` internally, enabling the same constant-state caching optimization while keeping behavior unchanged.

Benchmark was also adjusted to cache the salting part.

src/bench/crypto_hash.cpp

@@ -193,13 +193,11 @@ static void SHA512(benchmark::Bench& bench)
 static void SipHash_32b(benchmark::Bench& bench)
 {
     FastRandomContext rng{/*fDeterministic=*/true};
-    auto k0{rng.rand64()}, k1{rng.rand64()};
+    PresaltedSipHasher presalted_sip_hasher(rng.rand64(), rng.rand64());
     auto val{rng.rand256()};
     auto i{0U};
     bench.run([&] {
-        ankerl::nanobench::doNotOptimizeAway(SipHashUint256(k0, k1, val));
-        ++k0;
-        ++k1;
+        ankerl::nanobench::doNotOptimizeAway(presalted_sip_hasher(val));
         ++i;
         val.data()[i % uint256::size()] ^= i & 0xFF;
     });