perf(rust): optimize hash12 by directly hex-encoding 6-byte digest slices in ja4 and ja4x

[Aditya-9-6]

Summary

This PR optimizes the core hash12 helper function in both the ja4 and ja4x Rust crates.

Technical Details

Previously, hash12 performed hex::encode(Sha256::digest(s)) over all 32 bytes of the digest into a 64-character heap string, followed by sha256[..12].into() to extract the 12-char slice:

// Old: 2 heap allocations + hex encoding 26 unused trailing bytes
let sha256 = hex::encode(Sha256::digest(s));
sha256[..12].into()

[vlvkobal]

Let’s merge #312 first, then rebase this PR on top of it. @vvv, could you review the earlier pending PRs when you have a chance?

[vlvkobal]

One note for future PRs: a PR should have a clear idea behind it, and the changes in it should be coherent and related to that idea. Unrelated fixes, refactoring, and cleanup are better kept in separate PRs, otherwise the PR becomes harder to understand and review.

No need to split this one now, but please keep this in mind for future PRs.

[Aditya-9-6]

Yes sir !