fix(crypto): implement IHash64bit for Hash256AsKey

[JayeTurn]

Changes

• Implement IHash64bit<Hash256AsKey> for Hash256AsKey
• Add Hash256.GetHashCode64() backed by FastHash64For32Bytes
• Add a regression test covering SeqlockCache<Hash256AsKey, byte[]>

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• dotnet restore src/Nethermind/Nethermind.Core.Test/Nethermind.Core.Test.csproj --configfile /tmp/nethermind-nuget.config --verbosity minimal
• dotnet test --project src/Nethermind/Nethermind.Core.Test/Nethermind.Core.Test.csproj -c release --no-restore -- --filter FullyQualifiedName~Hash256AsKey_works_with_cache

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No

Remarks

• Aligns Hash256AsKey with the existing IHash64bit pattern already used by AddressAsKey for SeqlockCache.