feat: Add Mmr::nodes_from and Mmr::from_nodes_unchecked

[sergerad]

Describe your changes

Unblocks 0xMiden/node#2468 which wants to persist Mmr segments incrementally to a flat file.

Write side: adds Mmr::nodes_from(start), returning an iterator over the MMR's nodes at indices start.. in insertion (postorder) order. The node buffer is append-only, so a consumer that has persisted the first start nodes can sync incrementally by appending only the nodes returned here — this lets miden-node keep a flat file of 32-byte node records instead of rewriting the full MMR or using RocksDB.

Read side: adds Mmr::from_nodes_unchecked(forest, nodes), reconstructing an MMR from its complete postorder node array without re-hashing (the exact cost the flat file exists to avoid — try_from_iter recomputes every merge). The node count is validated against the forest, but node values are taken verbatim, so the caller is responsible for checking the result against a trusted commitment (e.g. peaks()) — same trust model as read_from_bytes, with the _unchecked suffix following Blockchain::from_mmr_unchecked. Deserializable::read_from now delegates to the same validation path instead of duplicating the count check.

The backing NodeStore::iter_from skips directly to the starting chunk. Tests cover chunk-boundary/mid-chunk/past-the-end starts, an incremental-persistence round trip, rebuild-from-nodes round trips (forest/nodes/peaks match and openings verify, including empty-forest and multi-chunk sizes), and count-mismatch rejection (MmrError::InvalidNodeCount).

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated CHANGELOG.md

[huitseeker]

Overall, this looks good, but I think this PR needs one code change and one wording change:

• Restore the early count check in read_from.
• State clearly that count validation checks representation shape, while trusted commitment comparison checks the accumulator state. from_nodes_unchecked does neither full rehashing nor full internal node validation.

[sergerad]

Just noting that this doesn't leak MmrNodeIter into pub api b/c pub(super) struct NodeStore.

[Al-Kindi-0]

LGTM

[huitseeker]

LGTM