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/test/fuzz/integer.cpp

@@ -118,7 +118,7 @@ FUZZ_TARGET(integer, .init = initialize_integer)
     }
     (void)MillisToTimeval(i64);
     (void)SighashToStr(uch);
-    (void)SipHashUint256(u64, u64, u256);
+    (void)PresaltedSipHasher(u64, u64)(u256);
     (void)PresaltedSipHasher(u64, u64)(u256, u32);
     (void)ToLower(ch);
     (void)ToUpper(ch);