cl/merkle_tree: add EIP-7916 progressive merkleization

[JackCC703]

Summary

• add MerkleizeProgressive following the recursive subtree layout defined by EIP-7916
• preserve the input chunks because MerkleizeVector hashes its input in place
• add pinned reference vectors covering progressive subtree boundaries

Motivation

This provides a reusable progressive Merkleization primitive for future ProgressiveList-based SSZ work, including EIP-7807.

Testing

• go test ./cl/merkle_tree -count=1

[domiwei]

Looks good — algorithm matches the EIP-7916 spec, reference vectors pass, and the in-place mutation guard is correct.

One optional suggestion: numLeaves * 4 can overflow uint64 to zero when numLeaves reaches 4^31 (2^62), which would cause infinite recursion. In practice this requires ~197 exabytes of input so it's not a real-world risk, but a one-line guard would close the theoretical path:

if numLeaves > math.MaxUint64/4 {
    return [32]byte{}, errors.New("progressive tree capacity overflow")
}

Non-blocking — approve as-is.

[JackCC703]

Looks good — algorithm matches the EIP-7916 spec, reference vectors pass, and the in-place mutation guard is correct.

One optional suggestion: numLeaves * 4 can overflow uint64 to zero when numLeaves reaches 4^31 (2^62), which would cause infinite recursion. In practice this requires ~197 exabytes of input so it's not a real-world risk, but a one-line guard would close the theoretical path:

if numLeaves > math.MaxUint64/4 {
    return [32]byte{}, errors.New("progressive tree capacity overflow")
}

Non-blocking — approve as-is.

Thanks for pointing this out.
I’ve added the overflow guard.

[Copilot]

Pull request overview

Adds a reusable progressive Merkleization primitive to cl/merkle_tree implementing the EIP-7916 “recursive subtree layout” (1, 4, 16, 64, … capacity subtrees, terminated by a zero chunk), along with pinned reference vectors to validate behavior across subtree boundary transitions.

Changes:

• Introduce MerkleizeProgressive that progressively merkleizes input chunks without mutating the caller’s slice (by copying per-subtree before calling MerkleizeVector).
• Add reference-vector tests covering progressive subtree boundary transitions and verifying input immutability.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cl/merkle_tree/progressive.go	Implements EIP-7916 progressive Merkleization with overflow protection and non-mutating behavior.
cl/merkle_tree/progressive_test.go	Adds pinned reference vectors and validates that MerkleizeProgressive does not modify input chunks.

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