feat: EIP-712-style typed structured data hashing & credential hash registry (#64)

[jotel-dev]

Description

Closes #64.

This PR implements an EIP-712-style typed structured data hashing scheme for StellarID credentials. It allows credentials to be verified off-chain (e.g. via QR code, NFC tap, or HTTP header) using only the credential fields and the issuer's public key, without performing any on-chain network queries.

Changes Made

• Data Model: Updated Credential struct to include pub credential_hash: BytesN<32>.
• Domain Separator: Defined deterministic domain separator formula: SHA-256("StellarID:v1:" || contract_address_32_bytes).
• Canonical Hash Encoding: Formatted fixed-width big-endian byte concatenation:
  SHA-256(domain_separator || schema_id_be || subject_32_bytes || issuer_32_bytes || issued_at_be || expires_at_be)
• Public Interface:
  • compute_expected_hash(env, schema_id, subject, issuer, issued_at, expires_at) -> BytesN<32> (pure helper for verifiers)
  • get_credential_hash(env, credential_id) -> BytesN<32>
  • verify_credential_hash(env, credential_id) -> bool
• Credential Lifecycle Integration: Updated issue_credential, batch_issue_credentials, bridge_credential, and issue_multisig_credential to compute and store credential hashes.
• Documentation: Updated docs/ARCHITECTURE.md with the canonical hashing specification.

Test Plan & Verification

• test_credential_hash_calculation_and_verification: Verifies that stored credential hashes match off-chain computed hashes.
• test_credential_hash_modifying_field_changes_hash: Verifies that altering any field changes the resulting hash.
• All unit tests passing via cargo test.