Switch SHA-256 implementation from libsodium to Rust.

[dmkozh]

Description

Rust implementation is faster because it uses SHA-NI instructions for hardware acceleration.

In a previous optimization iteration (never merged) we've used OpenSSL implementation in C++ (for the same reason), however it turned out to be problematic as some of its versions acquired lock for hashing, which made it non-parallelizable. That's why this accidentally got included into 'parallelization improvements' optimization batch - but I don't really see much harm in merging this sooner rather than later.

This has a tangible impact on apply load benchmarks (in the order of 10s of ms) and looks really good on micro-benchmarks (at least on my laptop, but most relatively modern CPUs should support SHA-NI):

libsodium
SHA256 bytes bench total duration: 26.6911 s, average duration per entry: 2.66911 ms
XDR SHA256 bench total duration: 42.8356 s, average duration per entry: 4.28356 ms

Rust
SHA256 bytes bench total duration: 4.57691 s, average duration per entry: 0.457691 ms
XDR SHA256 bench total duration: 15.4851 s, average duration per entry: 1.54851 ms

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

Moves SHA-256 hashing from libsodium to RustCrypto’s hardware-accelerated sha2 implementation.

Changes:

• Adds one-shot and incremental Rust SHA-256 APIs through the CXX bridge.
• Refactors C++ SHA-256 wrappers to use the Rust implementation.
• Adds and locks the sha2 dependency.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/rust/src/sha256.rs	Implements Rust SHA-256 hashing.
src/rust/src/lib.rs	Registers the SHA-256 module.
src/rust/src/bridge.rs	Exposes hashing APIs to C++.
src/rust/Cargo.toml	Adds the sha2 dependency.
src/crypto/SHA.h	Converts incremental state to PImpl.
src/crypto/SHA.cpp	Routes hashing through Rust.
Cargo.lock	Records the direct dependency.