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/Cargo.toml

@@ -83,7 +83,6 @@ zeroize = { version = "1.7", features = ["derive"] }
 # Mainline DHT (did:dht — BitTorrent DHT for decentralized identity)
 mainline = "2"
 zbase32 = "0.1"
-simple-dns = "0.7"
 
 # Systemd watchdog notification
 sd-notify = "0.4"