fix OOMs in Merkle/SMT deserialization

[huitseeker]

This fix OOMs from untrusted length prefixes, found through fuzzing.

The scheduled fuzz jobs miden-crypto (merkle) and miden-crypto (smt_serde) fail with allocation-size-too-big / OOM when random inputs encode absurd length prefixes. Both deserializers preallocate a Vec based on the prefix (Vec::with_capacity), which can request terabytes and abort. After those fixes, fuzzing exposes a pre-existing panic in PartialMerkleTree::with_leaves on empty input (unwrap() on None).

This is technically breaking since we now return an empty tree on depth 0 (instead of panicking).

The fix, how to verify

• prevents allocation attacks in Merkle/SMT deserializers by using read_many_iter and accurate
  min_serialized_size() bounds
• fixes PartialMerkleTree::with_leaves panic on empty input (return an empty tree instead)
• add budgeted-deserialization coverage for empty SMT/PMT/leaf and oversized PMT length

How to verify

• Run the fuzz targets that failed in CI:
  • cargo +nightly fuzz run merkle -- -max_total_time=60 -runs=10000
  • cargo +nightly fuzz run smt_serde -- -max_total_time=60 -runs=10000
• Run targeted tests:
  • cargo test -p miden-crypto merkle::partial_mt
  • cargo test -p miden-crypto merkle::smt
  • cargo test -p miden-crypto merkle::smt::full::tests::test_empty_smt_deserialization_with_budget

[krushimir]

There's an issue regarding deserialization of PartialMerkleTree due to NodeIndex alignment padding.

The tuple doesn't override min_serialized_size, so it falls back to the in-memory size including alignment padding.

For (NodeIndex, Word):

• in-memory: 48 bytes (NodeIndex has 7 bytes of padding between its u8 and u64 fields)
• serialized: 41 bytes (9 + 32, no padding)

The budget check divides remaining bytes by 48 instead of 41, rejecting valid input. This test fails on this branch:

#[test]
fn deserialize_nonempty_with_budget() {
    let mt = MerkleTree::new(VALUES8).unwrap();
    let ms = MerkleStore::from(&mt);
    let path33 = ms.get_path(mt.root(), NODE33).unwrap();
    let pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
    let bytes = pmt.to_bytes();
    let parsed = PartialMerkleTree::read_from_bytes_with_budget(&bytes, bytes.len()).unwrap();
    assert_eq!(pmt, parsed);
}

InvalidValue("requested 4 elements but reader can provide at most 3")

[huitseeker]

@krushimir Fixed in #827

[krushimir]

Verified the fix locally (cherry-picked from next) – works as expected. LGTM!

[bobbinth]

Looks good! Thank you!