feat: add did:dht support to verifiable credentials

- 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>

core/archipelago/src/identity_manager.rs

@@ -40,6 +40,9 @@ pub struct IdentityRecord {
     pub purpose: IdentityPurpose,
     pub pubkey_hex: String,
     pub did: String,
+    /// did:dht identifier (published to Mainline DHT for discoverability)
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub dht_did: Option<String>,
     pub created_at: String,
     /// Nostr secp256k1 public key in hex format
     pub nostr_pubkey: Option<String>,