jh: implement SerializableState

[Ap4sh]

part of #546

adds SerializableState for JhCore and exposes it through the public Jh224, Jh256, Jh384, and Jh512 wrappers via FixedHashTraits

the serialized core state stores the 128-byte compressor state plus the processed block count, matching the wrapper-level serialization tests used by the other hash crates

tests:

• cargo test -p jh --all-features
• cargo hack test --feature-powerset
• RUSTFLAGS='-Dwarnings' cargo +1.85.1 hack test --feature-powerset
• RUSTFLAGS='-Dwarnings' cargo +1.85.1 hack build --target thumbv7em-none-eabi --each-feature --exclude-features default,std
• RUSTFLAGS='-Dwarnings' cargo +1.85.1 hack build --target wasm32-unknown-unknown --each-feature --exclude-features default,std
• cargo fmt --all -- --check
• cargo clippy --all -- -D warnings
• typos

[newpavlov]

I would prefer to resolve #546 as well since it's unclear on the first glance that it's correct to use Compressor finalization like this, but I guess it's fine to merge it now.