Summary
- Add identity_store module with IdentityStore unit struct providing per-DID Keychain namespacing for multi-identity persistence
- Implement managed-dids index, per-DID device key generation (software + Secure Enclave paths), DID document and PLC audit log persistence
- 19 tests covering all acceptance criteria (AC2.1-AC2.9)
Test Plan
- All 19 automated tests pass (cargo test -p identity-wallet -- --test-threads=1 identity_store)
- Build, clippy, fmt all clean
- Human test plan at docs/test-plans/2026-03-28-plc-key-management.md (Secure Enclave verification on physical iOS device)