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/Cargo.toml

@@ -80,6 +80,11 @@ qrcode = "0.14"
 data-encoding = "2.6"
 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"