feat: Privacy-preserving identity aggregator with anonymous XOR accumulator set membership proofs (#65)

[jotel-dev]

Description

Closes #65.

This PR introduces an anonymous, privacy-preserving cryptographic accumulator for each credential schema. It allows wallet subjects to prove valid membership in a schema holder set using a snapshot witness without revealing their specific wallet address or credential ID.

Changes Made

• Storage Keys: Added DataKey::SchemaAccumulator(u32) and DataKey::SchemaHolderCount(u32).
• Accumulator Cryptography:
  • Accumulator formula: $\text{acc}{\text{new}} = \text{SHA-256}(\text{acc}{\text{old}} \oplus \text{SHA-256}(\text{subject_bytes}))$.
  • Initial state: 32 bytes of zeros (0x00...00).
• State Maintenance: Automatically updates schema accumulators and holder counts during issue_credential, batch_issue_credentials, bridge_credential, issue_multisig_credential, and revoke_credential.
• Public Interface:
  • get_schema_accumulator(env, schema_id) -> BytesN<32>
  • get_schema_holder_count(env, schema_id) -> u32
  • generate_membership_witness(env, subject, schema_id) -> (BytesN<32>, BytesN<32>)
  • verify_membership_witness(env, schema_id, subject_hash, snapshot) -> bool
• Documentation: Documented XOR accumulator mechanics in docs/ARCHITECTURE.md.

Test Plan & Verification

• test_schema_accumulator_issue_and_revoke: Verifies accumulator state transitions on credential issuance and revocation, witness generation, and snapshot verification.
• All unit tests passing via cargo test.