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>
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