feat: Selective disclosure proofs using Merkle trees of credential fields (#67)

[jotel-dev]

Description

Closes #67.

This PR adds selective disclosure capabilities to StellarID using Merkle tree proofs. Subjects can selectively disclose specific attributes (e.g., proving age: 30) to third-party verifiers without revealing other fields contained within the credential.

Changes Made

• Data Model: Added SelectiveCredential struct containing merkle_root: BytesN<32> and field_count.
• Storage Keys: Added DataKey::SelectiveCredential(u64) and DataKey::SelectiveCredentialCount.
• Merkle Cryptography:
  • Leaf Hash: SHA-256(field_name_xdr || ":" || field_value_xdr).
  • Binary Tree Construction: Pairs leaves and duplicates odd leaf layers to compute merkle_root.
  • Proof Verification: Recomputes Merkle path from proof elements and checks against stored root.
• Public Interface:
  • hash_leaf(env, field_name, field_value) -> BytesN<32>
  • compute_merkle_root(env, leaves) -> BytesN<32>
  • verify_merkle_proof(env, leaf, proof, root, index) -> bool
  • issue_selective_credential, get_selective_credential, verify_credential_field, has_valid_field.
• Documentation: Added Merkle tree selective disclosure documentation to docs/ARCHITECTURE.md.

Test Plan & Verification

• test_selective_disclosure_merkle_tree: Builds a 4-leaf Merkle tree credential, generates sibling proofs, and verifies individual field disclosure on-chain.
• All unit tests passing via cargo test.