feat: open LargeSmtForest backend API for external implementations

[JereSalo]

Description

The Backend trait for LargeSmtForest is public but cannot be implemented outside this crate, because compute_mutations and apply_mutations must return LineageMutation and AppliedLineageMutation values whose fields are private and whose constructors are pub(crate). This PR makes both constructors public and adds a LineageId::as_bytes getter so backends can persist lineage identifiers.

It also adds a MutationSet::from_parts constructor. An external backend has to produce the forward and reverse mutation sets carried by AppliedLineageMutation, and since MutationSet fields are private the only way to build them today is to load the full tree into memory and run Smt::compute_mutations / apply_mutations_with_reversion, which makes update cost scale with tree size instead of change size. With the constructor, a backend can compute mutations against its persisted tree data (affected leaves and stored subtree nodes only) and assemble both sets directly, the same way the built-in RocksDB backend does internally. In the SQLite backend prototype this took a single-entry committed update on a 1,000,000-entry tree from ~30 s to ~1 ms.

We need this change because in 0xMiden/rust-sdk#2315 we want to use the LargeSmtForest but with an SQLite backend. And it's very likely that we'll want to do the same in the web SDK with IndexedDB.

In order to make use of this change we should also bump miden-crypto, so it'd be good to have this in 0.28.1 since miden-protocol currently pins ^0.28.

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[JereSalo]

Ideally we would like this in crypto 0.28.1 so we can get to use this soon. Since crypto has been moved to this repository I'm not entirely sure how crypto releases are managed. I pointed this PR to next, but if you need me to make any changes regarding that let me know.

[Al-Kindi-0]

LGTM

[huitseeker]

This works! This will get published as 0.28.1 and does not need a major bump.

[huitseeker]

@JereSalo This will normalize as soon as we release the VM at 0.28, but what you did is just fine.

[JereSalo]

Thanks for the review. I had to add one more little change that I noticed was necessary while implementing the LargeSmtForest Backend in 0xMiden/rust-sdk#2333. It's just a constructor. I updated the PR description accordingly. Let me know what you think.