feat(merkle): implement inclusion proof generation and verification

[s8sato]

I've split out the cryptographic part of the trigger execution logs (from #4968) into a separate PR: #5467

Summary

This PR introduces Merkle proof generation and verification to support inclusion checking, as a prerequisite for #4968 (and possibly #4637). It also includes a fix to the construction of the Merkle tree to ensure correct transaction indexing.

Changes

Features

• Added MerkleProof<T> struct that includes:
  • leaf_index: the position of the leaf in the tree
  • sibling_hashes: the list of sibling hashes from the leaf up to the root (excluding the root itself)
• Implemented MerkleTree::get_proof(leaf_index) to generate an inclusion proof
• Implemented MerkleProof::verify(root, max_height) to verify the proof against a given root

Fixes

• Fixed the binary operation used to compute parent hashes from child hashes: it was previously commutative (addition), but has been corrected to a non-commutative (ordered) operation. The previous implementation could not preserve transaction index ordering guarantees.

Notes

• max_height is passed explicitly to verify() to avoid trusting unbounded proof data.
• The Merkle proof format and verification logic follow a standard binary Merkle tree structure. See:
  • RFC 6962: Certificate Transparency Merkle Trees
  • BSV Technical Standards Committee – Merkle Proof Standardised Format

Related

• Closes part of #4968
• Prepares groundwork for cross-chain interoperability: #4637