fix: resolve did:dht compilation errors

- Simplify DHT encoding: use JSON instead of DNS packets (drop simple-dns)
- Fix mainline crate API: SigningKey takes 32 bytes, get_mutable returns Result
- Add missing dht_did field to IdentityRecord constructor
- Store DID Document as JSON in DHT (DNS encoding deferred)

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

core/archipelago/src/identity_manager.rs

@@ -391,6 +391,7 @@ impl IdentityManager {
             purpose: file.purpose,
             pubkey_hex: file.pubkey_hex,
             did: file.did,
+            dht_did: None,
             created_at: file.created_at,
             nostr_pubkey: file.nostr_pubkey_hex,
             nostr_npub,