feat: implement did:dht creation and resolution via Mainline DHT

DHT-02: did:dht creation
- network/did_dht.rs: z-base-32 encoding, DNS packet encoding, BEP-44
  mutable item publication via mainline crate
- identity.create-dht-did RPC endpoint
- dht_did field added to IdentityRecord
- get_signing_key() exposed on IdentityManager

DHT-03: did:dht resolution
- did_dht::resolve() queries DHT, parses DNS → DID Document
- DhtDidCache with 1-hour TTL
- identity.resolve-dht-did, identity.refresh-dht-did, identity.dht-status

New dependencies: mainline 2, zbase32 0.1, simple-dns 0.7

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/identity_manager.rs

@@ -210,6 +210,11 @@ impl IdentityManager {
         Ok(())
     }
 
+    /// Get the Ed25519 signing key for an identity (for DHT publication).
+    pub async fn get_signing_key(&self, id: &str) -> Result<SigningKey> {
+        self.load_signing_key(id).await
+    }
+
     /// Sign data with a specific identity.
     pub async fn sign(&self, id: &str, data: &[u8]) -> Result<String> {
         let signing_key = self.load_signing_key(id).await?;