- Add dht_did field to IdentityRecord (optional, serde-compatible)
- Add prefer_dht_did param to identity.issue-credential RPC
- When true and dht_did is set, uses did:dht as VC issuer
- Credential system already format-agnostic for any DID type
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>